gdb: Make GMP a required dependency for building GDB

This commit modifies gdb's configure script to trigger an error
if we cannot find a usable libgmp.

For the record, making this a requirement was discussed in March 2018:
https://sourceware.org/pipermail/gdb-patches/2018-March/147373.html

gdb/ChangeLog:

        * configure.ac: Generate an error if a usable GMP library
        could not be found.
        * configure: Regenerate.

gdb/configure.ac

@@ -687,6 +687,9 @@ fi
 AC_LIB_HAVE_LINKFLAGS([gmp], [], [#include <gmp.h>],
                       [mpz_t n;
                        mpz_init (n);])
+if test "$HAVE_LIBGMP" != yes; then
+  AC_MSG_ERROR([GMP is missing or unusable])
+fi
 
 AC_ARG_WITH(mpfr,
   AS_HELP_STRING([--with-mpfr], [include MPFR support (auto/yes/no)]),