Check for -Wno-unused-but-set-variable gcc support

Gcc versions 4.3.2 and earlier do not support the compiler flag
-Wno-unused-but-set-variable.  This can lead to build failures
on older Linux platforms such as Debian Lenny.  Since this is
an optional build argument this changes add a new autoconf check
for the option.  If it is supported by the installed version of
gcc then it is used otherwise it is omited.

See commit's 12c1acde76 and
79713039a2 for the reason the
-Wno-unused-but-set-variable options was originally added.

config/Rules.am

@@ -2,7 +2,7 @@ DEFAULT_INCLUDES = -include ${top_builddir}/zfs_config.h
 
 AM_LIBTOOLFLAGS = --silent
 AM_CFLAGS  = -Wall -Wstrict-prototypes
-AM_CFLAGS += -fno-strict-aliasing -Wno-unused-but-set-variable ${DEBUG_CFLAGS}
+AM_CFLAGS += -fno-strict-aliasing ${NO_UNUSED_BUT_SET_VARIABLE} ${DEBUG_CFLAGS}
 AM_CFLAGS += -D_GNU_SOURCE -D__EXTENSIONS__ -D_REENTRANT
 AM_CFLAGS += -D_POSIX_PTHREAD_SEMANTICS -D_FILE_OFFSET_BITS=64
 AM_CFLAGS += -D_LARGEFILE64_SOURCE -DTEXT_DOMAIN=\"zfs-linux-user\"

config/kernel.m4

@@ -46,7 +46,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 
 	dnl # -Wall -fno-strict-aliasing -Wstrict-prototypes and other
 	dnl # compiler options are added by the kernel build system.
-	KERNELCPPFLAGS="$KERNELCPPFLAGS -Wno-unused-but-set-variable"
+	KERNELCPPFLAGS="$KERNELCPPFLAGS $NO_UNUSED_BUT_SET_VARIABLE"
 	KERNELCPPFLAGS="$KERNELCPPFLAGS -DHAVE_SPL -D_KERNEL"
 	KERNELCPPFLAGS="$KERNELCPPFLAGS -DTEXT_DOMAIN=\\\"zfs-linux-kernel\\\""
 

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

@@ -0,0 +1,27 @@
+dnl #
+dnl # Check if gcc supports -Wno-unused-but-set-variable option.
+dnl #
+dnl # We actually invoke gcc with the -Wunused-but-set-variable option
+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_MSG_CHECKING([for -Wno-unused-but-set-variable support])
+
+	saved_flags="$CFLAGS"
+	CFLAGS="$CFLAGS -Wunused-but-set-variable"
+
+	AC_RUN_IFELSE(AC_LANG_PROGRAM( [], []),
+	[
+		NO_UNUSED_BUT_SET_VARIABLE=-Wno-unused-but-set-variable
+		AC_MSG_RESULT([yes])
+	],
+	[
+		NO_UNUSED_BUT_SET_VARIABLE=
+		AC_MSG_RESULT([no])
+	])
+
+	CFLAGS="$saved_flags"
+	AC_SUBST([NO_UNUSED_BUT_SET_VARIABLE])
+])

config/user.m4

@@ -9,5 +9,6 @@ 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
 ])