configure: Find correct libgsm's gsm.h header

Some distributions (archlinux) or versions of libgsm install gsm.h in
/usr/include/gsm/gsm.h

Since libgsm doesn't come with a pkfconfig, let's first check if gsm.h
and take the correct path in the build setup.

Change-Id: I07d3c03903e0d4bb80e843c7ed917a27b791ea53
2 files changed