[gdb/build] Handle deprecation of scm_install_gmp_memory_functions

When building gdb with guile 3.0.8, we run into:
...
gdb/guile/guile.c: In function \
  'void gdbscm_initialize(const extension_language_defn*)':
gdb/guile/guile.c:680:5: error: 'scm_install_gmp_memory_functions' is \
  deprecated [-Werror=deprecated-declarations]
  680 |     scm_install_gmp_memory_functions = 0;
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/guile/3.0/libguile.h:128,
                 from gdb/guile/guile-internal.h:30,
                 from gdb/guile/guile.c:36:
/usr/include/guile/3.0/libguile/deprecated.h:164:20: note: declared here
  164 | SCM_DEPRECATED int scm_install_gmp_memory_functions;
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[1]: *** [Makefile:1896: guile/guile.o] Error 1
...

The variable has been deprecated because it no longer has any effect.

Fix this by disabling the specific deprecation warning.

Also handle upcoming guile versions > 3.0, in which the variable will be
removed, by limiting the usage of the variable to guile versions <= 3.0.

This does not break anything.  The variable was merely used to address a
problem present in guile versions <= v3.0.5.

Note that we don't limit the usage of the variable to guile versions <= 3.0.5,
because we want to support f.i. building against 3.0.6 and then using a shared
lib with 3.0.5.

Tested on x86_64-linux.

Co-Authored-By: Tom de Vries <tdevries@suse.de>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28994