Hi,

this enables GDB build with in-tree GMP and MPFR.

This updated version of my patch added support for in-tree MPFR.

While there is no preexisting --with-gmp configure option in gdb,
this leaves the current --with-mpfr=yes/no/auto as it is, since it seems
to work by accident.  The top level config script thinks "no" is
a DIR, and skips the detection of in-tree MPFR in that case, while
gdb honors --with-mpfr=no, so that is what's expected.

The in-tree build works only when no --with-mpfr and no
--with-gmp is given.  While it does not advertise --with-gmp-prefix=DIR
and/or --with-mpfr-prefix=DIR with "./configure --help", I'll leave that
for another patch.


Tested on x86_64-pc-linux-gnu and cross-build for arm.
Is it OK for the trunk?


Thanks
Bernd.