produce robust makefiles #864

When compiling, CC options and library path and flags are resolved during the execution of nitg.
Thus the generated makefile and cie depends on the current system and libraries instead of being generic.
This reduce the universality of #860 .

Eg. pkgconfig should be invoked by the generated Makefile instead of nitg. So the correct options will depend on the final system instead of the original one.

This will also make the primitive compiler (c_src) more portable and robust.

• use pkg-config in Makefile to locate libgc (solved by Bugfixes for MacOSX #865)
• use pkg-confile in Makefile for the pkgconfir annotation (proposed by Pkgconfig #874)
• portable stacktraces
• move exec in c_compiler_option and c_linker_option to the Makefile