package/gdb: fix when gmp is installed on the host machine

gdb's AC_LIB_HAVE_LINKFLAGS macro hardcodes a search starting with
/usr/lib/ which can lead to libgmp from the wrong architecture with the
result that the test fails.  Even if a libgmp is found there it is now
the one that should be used.

This is the same macro used for expat and lzma for which there are
already specific CONF_OPTS flags added here.  Add the same flag for
libgmp and move the handling down so that it is logically grouped with
the other similar options.

Note that there is no --with(out)-gmp flag to configure, as the
dependency is mandatory, so only the --with-libgmp-prefix option is
specified.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 9fa5d641ac4951087bb007e091a799101d294527)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/gdb/gdb.mk

@@ -77,14 +77,6 @@ ifeq ($(BR2_GDB_VERSION_11),y)
 HOST_GDB_DEPENDENCIES += host-gmp
 endif
 
-# When BR2_GDB_VERSION_11=y (because it's enabled for the host) and
-# we're building the full gdb for the target, we need gmp as a
-# dependency. For now the default gdb version in Buildroot doesn't
-# require gmp.
-ifeq ($(BR2_GDB_VERSION_11)$(BR2_PACKAGE_GDB_DEBUGGER),yy)
-GDB_DEPENDENCIES += gmp
-endif
-
 # When gdb sources are fetched from the binutils-gdb repository, they
 # also contain the binutils sources, but binutils shouldn't be built,
 # so we disable it (additionally the option --disable-install-libbfd
@@ -169,6 +161,16 @@ GDB_CONF_OPTS += \
 	--without-curses
 endif
 
+# When BR2_GDB_VERSION_11=y (because it's enabled for the host) and
+# we're building the full gdb for the target, we need gmp as a
+# dependency. For now the default gdb version in Buildroot doesn't
+# require gmp.
+ifeq ($(BR2_GDB_VERSION_11)$(BR2_PACKAGE_GDB_DEBUGGER),yy)
+GDB_CONF_OPTS += \
+	--with-libgmp-prefix=$(STAGING_DIR)/usr
+GDB_DEPENDENCIES += gmp
+endif
+
 ifeq ($(BR2_PACKAGE_GDB_SERVER),y)
 GDB_CONF_OPTS += --enable-gdbserver
 GDB_DEPENDENCIES += $(TARGET_NLS_DEPENDENCIES)