Detect gmp using pkgconfig

[ClementPernet]

Now that GMP is also using pkgconfig files, givaro could simplify its detection scripts by invoquing somehting like
PKG_CHECK_MODULES([GMPXX],[gmpxx >= X.X.X])

[dlesnoff]

On Jean-Zay (french supercomputer), I was not able to use the latest gmp-6.3.0 version.
Even with the path specified in the configure call:

./configure --with-gmp=$HOME/gmp-6.3.0/

the config.log had gmp-6.2.1 in configured path.
The version corresponds to the gmp module I loaded and unloaded.
I guess the directory to gmp-6.2.1 (inside a gcc's folder) was kept and not erased by module unload.

The only workaround I found was to reload the gcc module.
Even when specifying the library and include directories separately, my gmp.h file is never found.

This feature is thus really desired. I might look into it after January.