edit ligtirb.pc to make gtirb usage easier

[gogo2464]

Currently when we want to use gtirb from Makefile, we have to firstt set the variable export PKG_CONFIG_PATH= to point to the folder pkgconfig and to edit our own file such as:

libgtirb.pc:
prefix=/usr/lib
exec_prefix=${prefix}
includedir=${prefix}/include
libdir=${exec_prefix}/lib

Name: libgtirb
Description: The foo library
Version: 1.0.0
Cflags: -I${includedir}/libgtirb
Libs: -L${libdir} -llibgtirb

It is not easy to use. I suggest to edit Makefile to autogenerate these.