you need to download the HLSDK and Metamod sources (as stefan said - they can be found at
http://www.metamod.org ), and then adapt the Makefile (change the compiler command name and the paths to HLSDK and Metamod sources) then running "make" will produce the .so file if nothing is wrong.
to sum up, these are the lines which you need to change in Makefile:
CPP =
gcc-2.95.3
METAMOD_SRCDIR =
/usr/src/hl_bot_build/metamod-1.17/metamod
HLSDK_BASEDIR =
/usr/src/hl_bot_build/hlsdk-2.3