gdb/configure: Add --with-libgmp-prefix option

This patch allows a user to tell gdb's configure script where
his GMP library is installed.

gdb/ChangeLog:

        * configure.ac: Add support for --with-libgmp-prefix.
        * Makefile.in (LIBGMP): New variable.
        (CLIBS): Include $(LIBGMP).
        * configure, config.in: Regenerate

gdb/configure.ac

@@ -683,6 +683,11 @@ else
   fi
 fi
 
+# Verify that we have a usable GMP library.
+AC_LIB_HAVE_LINKFLAGS([gmp], [], [#include <gmp.h>],
+                      [mpz_t n;
+                       mpz_init (n);])
+
 AC_ARG_WITH(mpfr,
   AS_HELP_STRING([--with-mpfr], [include MPFR support (auto/yes/no)]),
   [], [with_mpfr=auto])