header include path

where exactly is clanlib on your system?

as for picking up headers in non-standard locations, the CFLAGS environment variable takes care of that (and setting compiler optimizations and such) so for example if the header is in /usr/local/include/ClanLib-0.7/ClanLib you could:

export CFLAGS="-I/usr/local/include/ClanLib-0.7/ClanLib"

and it should pick up that directory

for the libs in nonstandard directories LDFLAGS or LD_LIBRARY_PATH will do (I like the latter better)
so for example if clanlib was installed in /usr/local/lib/clanlib you could:

export LD_LIBRARY_PATH=/usr/local/lib/clanlib:$LD_LIBRARY_PATH

which will set /usr/local/lib/clanlib as the first directory searched by the linker.