Makefiles are deprecated, it says so on the very page you linked.
Here is the steps I use to build the sample extension, SDK is l4d2 only because that's what I have at hand.
PHP Code:
alliedmodders/sourcemod/public/sample_ext (git)-[master] % pwd
/home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/sample_ext
alliedmodders/sourcemod/public/sample_ext (git)-[master] % mkdir build && cd build
alliedmodders/sourcemod/public/sample_ext/build (git)-[master] % export CC=clang CXX=clang++
alliedmodders/sourcemod/public/sample_ext/build (git)-[master] % python3 ../configure.py --sdks=l4d2
Checking CC compiler (vendor test gcc)... ['clang', 'test.c', '-o', 'test']
found clang version 11.1
Checking CXX compiler (vendor test gcc)... ['clang++', 'test.cpp', '-o', 'testp']
found clang version 11.1
alliedmodders/sourcemod/public/sample_ext/build (git)-[master] % ambuild
[create] mkdir -p sample.ext.2.l4d2
[create] mkdir -p package
[create] mkdir -p package/addons
[create] mkdir -p package/addons/sourcemod
[create] mkdir -p package/addons/sourcemod/extensions
Spawned worker (pid: 316471)
Spawned worker (pid: 316472)
Spawned worker (pid: 316473)
Spawned worker (pid: 316474)
[316473] ln -s "/home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/lib/linux/libtier0_srv.so" "./sample.ext.2.l4d2/libtier0_srv.so"
[316473] ln -s "/home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/lib/linux/libvstdlib_srv.so" "./sample.ext.2.l4d2/libvstdlib_srv.so"
[316472] clang++ -pipe -fno-strict-aliasing -Wall -Werror -Wno-unused -Wno-switch -Wno-array-bounds -msse -m32 -fvisibility=hidden -Wno-narrowing -Wno-sometimes-uninitialized -g3 -std=c++14 -fno-exceptions -fno-threadsafe-statics -Wno-non-virtual-dtor -Wno-overloaded-virtual -fvisibility-inlines-hidden -Wno-inconsistent-missing-override -Wno-delete-non-virtual-dtor -Wno-implicit-exception-spec-mismatch -Wno-deprecated-register -Dstricmp=strcasecmp -D_stricmp=strcasecmp -D_snprintf=snprintf -D_vsnprintf=vsnprintf -DHAVE_STDINT_H -DGNUC -D_LINUX -DPOSIX -DSE_EPISODEONE=1 -DSE_ORANGEBOX=3 -DSE_CSS=6 -DSE_HL2DM=7 -DSE_DODS=8 -DSE_SDK2013=9 -DSE_TF2=11 -DSE_LEFT4DEAD=12 -DSE_NUCLEARDAWN=13 -DSE_LEFT4DEAD2=15 -DSE_DARKMESSIAH=2 -DSE_ALIENSWARM=16 -DSE_BLOODYGOODTIME=4 -DSE_EYE=5 -DSE_CSGO=21 -DSE_PORTAL2=17 -DSE_BLADE=18 -DSE_INSURGENCY=19 -DSE_CONTAGION=14 -DSE_BMS=10 -DSE_DOI=20 -DSOURCE_ENGINE=15 -DCOMPILER_GCC -DNO_HOOK_MALLOC -DNO_MALLOC_OVERRIDE -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/sample_ext -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/sample_ext/sdk -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/extensions -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/sourcepawn/include -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/amtl/amtl -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/amtl -I /home/l4d2/l4d2-buildtest/alliedmodders/mmsource-1.10/core -I /home/l4d2/l4d2-buildtest/alliedmodders/mmsource-1.10/core/sourcehook -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/engine -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/mathlib -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/vstdlib -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/tier0 -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/tier1 -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/game/server -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/toolframework -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/game/shared -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/common -H -c /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/smsdk_ext.cpp -o _home_l4d2_l4d2_buildtest_alliedmodders_sourcemod_public_smsdk_ext.o
[316471] clang++ -pipe -fno-strict-aliasing -Wall -Werror -Wno-unused -Wno-switch -Wno-array-bounds -msse -m32 -fvisibility=hidden -Wno-narrowing -Wno-sometimes-uninitialized -g3 -std=c++14 -fno-exceptions -fno-threadsafe-statics -Wno-non-virtual-dtor -Wno-overloaded-virtual -fvisibility-inlines-hidden -Wno-inconsistent-missing-override -Wno-delete-non-virtual-dtor -Wno-implicit-exception-spec-mismatch -Wno-deprecated-register -Dstricmp=strcasecmp -D_stricmp=strcasecmp -D_snprintf=snprintf -D_vsnprintf=vsnprintf -DHAVE_STDINT_H -DGNUC -D_LINUX -DPOSIX -DSE_EPISODEONE=1 -DSE_ORANGEBOX=3 -DSE_CSS=6 -DSE_HL2DM=7 -DSE_DODS=8 -DSE_SDK2013=9 -DSE_TF2=11 -DSE_LEFT4DEAD=12 -DSE_NUCLEARDAWN=13 -DSE_LEFT4DEAD2=15 -DSE_DARKMESSIAH=2 -DSE_ALIENSWARM=16 -DSE_BLOODYGOODTIME=4 -DSE_EYE=5 -DSE_CSGO=21 -DSE_PORTAL2=17 -DSE_BLADE=18 -DSE_INSURGENCY=19 -DSE_CONTAGION=14 -DSE_BMS=10 -DSE_DOI=20 -DSOURCE_ENGINE=15 -DCOMPILER_GCC -DNO_HOOK_MALLOC -DNO_MALLOC_OVERRIDE -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/sample_ext -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/sample_ext/sdk -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/extensions -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/sourcepawn/include -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/amtl/amtl -I /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/amtl -I /home/l4d2/l4d2-buildtest/alliedmodders/mmsource-1.10/core -I /home/l4d2/l4d2-buildtest/alliedmodders/mmsource-1.10/core/sourcehook -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/engine -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/mathlib -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/vstdlib -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/tier0 -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/tier1 -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/game/server -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/public/toolframework -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/game/shared -I /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/common -H -c /home/l4d2/l4d2-buildtest/alliedmodders/sourcemod/public/sample_ext/extension.cpp -o extension.o
[316472] clang++ _home_l4d2_l4d2_buildtest_alliedmodders_sourcemod_public_smsdk_ext.o extension.o libvstdlib_srv.so libtier0_srv.so -m32 -Wl,--exclude-libs,ALL -lm -lgcc_eh /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/lib/linux/tier1_i486.a /home/l4d2/l4d2-buildtest/alliedmodders/hl2sdk-l4d2/lib/linux/mathlib_i486.a -shared -o sample.ext.2.l4d2.so
[316472] cp "sample.ext.2.l4d2/sample.ext.2.l4d2.so" "./package/addons/sourcemod/extensions/sample.ext.2.l4d2.so"
Build succeeded.
Linear Mode
