Always check -Wno-unused-but-set-variable gcc support

The previous commit 8a7e1ceefa wasn't
quite right.  This check applies to both the user and kernel space
build and as such we must make sure it runs regardless of what
the --with-config option is set too.

For example, if --with-config=kernel then the autoconf test does
not run and we generate build warnings when compiling the kernel
packages.

config/always-no-unused-but-set-variable.m4

@@ -6,7 +6,7 @@ dnl # and infer the 'no-' version does or doesn't exist based upon
 dnl # the results.  This is required because when checking any of
 dnl # no- prefixed options gcc always returns success.
 dnl #
-AC_DEFUN([ZFS_AC_CONFIG_USER_NO_UNUSED_BUT_SET_VARIABLE], [
+AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_NO_UNUSED_BUT_SET_VARIABLE], [
 	AC_MSG_CHECKING([for -Wno-unused-but-set-variable support])
 
 	saved_flags="$CFLAGS"

config/user.m4

@@ -9,6 +9,5 @@ AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_LIBBLKID
 	ZFS_AC_CONFIG_USER_LIBSELINUX
 	ZFS_AC_CONFIG_USER_FRAME_LARGER_THAN
-	ZFS_AC_CONFIG_USER_NO_UNUSED_BUT_SET_VARIABLE
 	ZFS_AC_CONFIG_USER_STACK_GUARD
 ])

config/zfs-build.m4

@@ -33,6 +33,10 @@ AC_DEFUN([ZFS_AC_DEBUG], [
 	AC_MSG_RESULT([$enable_debug])
 ])
 
+AC_DEFUN([ZFS_AC_CONFIG_ALWAYS], [
+	ZFS_AC_CONFIG_ALWAYS_NO_UNUSED_BUT_SET_VARIABLE
+])
+
 AC_DEFUN([ZFS_AC_CONFIG], [
 	TARGET_ASM_DIR=asm-generic
 	AC_SUBST(TARGET_ASM_DIR)
@@ -47,6 +51,8 @@ AC_DEFUN([ZFS_AC_CONFIG], [
 	AC_MSG_RESULT([$ZFS_CONFIG]);
 	AC_SUBST(ZFS_CONFIG)
 
+	ZFS_AC_CONFIG_ALWAYS
+
 	case "$ZFS_CONFIG" in
 		kernel) ZFS_AC_CONFIG_KERNEL ;;
 		user)	ZFS_AC_CONFIG_USER   ;;