debug/gdb: add option to use GMP and MPFR

GMP and MPFR are optional for gdb, so offer a config option.

config/debug/gdb.in.cross

@@ -10,10 +10,27 @@ config GDB_CROSS
 
 if GDB_CROSS
 
+config GDB_CROSS_USE_GMP_MPFR
+    bool
+    prompt "Use GMP/MPFR"
+    default n
+    select GMP
+    select MPFR
+    select GDB_CROSS_NO_STATIC
+    help
+      gdb can use GMP+MPFR, although it does not seem compulsory.
+      
+      Say 'Y' here if you want to use GMP+MPFR.
+
+config GDB_CROSS_NO_STATIC
+    bool
+    default n
+
 config GDB_CROSS_STATIC
     bool
     prompt "Build a static cross gdb"
     default n
+    depends on ! GDB_CROSS_NO_STATIC
     help
       A static cross gdb can be usefull if you debug on a machine that is
       not the one that is used to compile the toolchain.

scripts/build/debug/300-gdb.sh

@@ -99,8 +99,10 @@ do_debug_gdb_build() {
         cd "${CT_BUILD_DIR}/build-gdb-cross"
 
         cross_extra_config=("${extra_config[@]}")
-        [ -z "${CT_GMP}"    ] || cross_extra_config+=("--with-gmp=${CT_PREFIX_DIR}")
-        [ -z "${CT_MPFR}"   ] || cross_extra_config+=("--with-mpfr=${CT_PREFIX_DIR}")
+        if [ "${CT_GDB_CROSS_USE_GMP_MPFR}" = "y" ]; then
+            cross_extra_config+=("--with-gmp=${CT_PREFIX_DIR}")
+            cross_extra_config+=("--with-mpfr=${CT_PREFIX_DIR}")
+        fi
         case "${CT_THREADS}" in
             none)   cross_extra_config+=("--disable-threads");;
             *)      cross_extra_config+=("--enable-threads");;