libgmp: Avoid warning when --without-libgmp is used

[Bruno Haible]

After creating a testdir of module libgmp
  ./gnulib-tool --create-testdir --dir=/tmp/testdir --single-configure libgmp
and configuring it with --without-libgmp, I get a GCC warning:

gcc -DHAVE_CONFIG_H -I. -I..  -DGNULIB_STRICT_CHECKING=1   -g -O2 -MT 
mini-gmp-gnulib.o -MD -MP -MF .deps/mini-gmp-gnulib.Tpo -c -o mini-gmp-gnulib.o 
mini-gmp-gnulib.c
mini-gmp-gnulib.c:28:10: warning: unknown option after ‘#pragma GCC diagnostic’ 
kind [-Wpragmas]
 # pragma GCC diagnostic ignored "-Wsuggest-attribute=malloc"
          ^

GCC 8 is the first version that supports
'# pragma GCC diagnostic ignored "-Wsuggest-attribute=malloc"'.


2020-07-12  Bruno Haible  <bruno@clisp.org>

        libgmp: Avoid warning when --without-libgmp is used.
        * lib/mini-gmp-gnulib.c: Ignore -Wsuggest-attribute=malloc only for
        GCC >= 8, not for GCC >= 4.6.

diff --git a/lib/mini-gmp-gnulib.c b/lib/mini-gmp-gnulib.c
index 5019be5..e9e8a17 100644
--- a/lib/mini-gmp-gnulib.c
+++ b/lib/mini-gmp-gnulib.c
@@ -22,12 +22,14 @@
 
 #include "mini-gmp.h"
 
-/* Pacify GCC -Wsuggest-attribute=const, malloc, pure.  */
+/* Pacify GCC -Wsuggest-attribute=const, pure, malloc.  */
 #if 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
 # pragma GCC diagnostic ignored "-Wsuggest-attribute=const"
-# pragma GCC diagnostic ignored "-Wsuggest-attribute=malloc"
 # pragma GCC diagnostic ignored "-Wsuggest-attribute=pure"
 #endif
+#if 8 <= __GNUC__
+# pragma GCC diagnostic ignored "-Wsuggest-attribute=malloc"
+#endif
 
 /* Pacify GCC -Wunused-variable for variables used only in 'assert' calls.  */
 #if defined NDEBUG && 4 < __GNUC__ + (6 <= __GNUC_MINOR__)