Build using --with-gmp and shared libraries

[François-Xavier Coudert]

>> Here is a patch to fix PR libfortran/21547: when building with
>> --with-gmp=/foo/bar and a shared libgmp in /foo/bar, the
>> $(RPATH_ENVVAR) variable (usually LD_LIBRARY_PATH) is not set
>> correctly when using the freshly built gfortran to build libgfortran.
>> The same thing happens for the gfortran testsuite, and the fix is also
>> included in this patch.
>
> Why wouldn't mpfr have the same problem?

Sorry not to mention that. It can (although it usually doesn't,
because the default for libmpfr is to build only a static library),
and this is fix by the same patch. In fact, in the gcc configury, all
occurences of GMPLIBS, GMPINCS always take care of both gmp and mpfr.
I followed this lead, and GMPLIBSDIR points to the directories where
gmp and mpfr are installed, if different. All this is done in the
toplevel configure.

FX