btrfs-progs: build: fix detection of std=gnu11 build

Commit 0d5767e664 ("btrfs-progs: build: use -std=gnu11") was supposed
to set C level to gnu11 to match kernel but it was done only in the C=1
(checker) branch. The std used was still gnu90 that can be verified by
running with V=1.

Set the std option to gnu11 unconditionally and delete the m4 macros
that were detecting support on older gcc (4.5). The reference build on
lowest supported distro (Centos 7) compiles fine.

Signed-off-by: David Sterba <dsterba@suse.com>

Makefile

@@ -88,7 +88,7 @@ ENABLE_WARNING_FLAGS := $(call cc-option, -Wimplicit-fallthrough)
 
 # Common build flags
 CFLAGS = $(SUBST_CFLAGS) \
-	 $(CSTD) \
+	 -std=gnu11 \
 	 -include config.h \
 	 -DBTRFS_FLAT_INCLUDES \
 	 -D_XOPEN_SOURCE=700  \
@@ -107,7 +107,7 @@ CFLAGS = $(SUBST_CFLAGS) \
 	 $(EXTRA_CFLAGS)
 
 LIBBTRFSUTIL_CFLAGS = $(SUBST_CFLAGS) \
-		      $(CSTD) \
+		      -std=gnu11 \
 		      -D_GNU_SOURCE \
 		      -fPIC \
 		      -fvisibility=hidden \
@@ -415,7 +415,6 @@ ifdef C
 			grep -v __SIZE_TYPE__ > $(check_defs))
 	check = $(CHECKER)
 	check_echo = echo
-	CSTD = -std=gnu11
 else
 	check = true
 	check_echo = true

Makefile.inc.in

@@ -4,7 +4,6 @@
 export
 
 CC = @CC@
-CSTD = @BTRFS_CSTD_FLAGS@
 LN_S = @LN_S@
 AR = @AR@
 RM = @RM@

configure.ac

@@ -38,7 +38,6 @@ AC_CONFIG_SRCDIR([btrfs.c])
 AC_PREFIX_DEFAULT([/usr/local])
 
 AC_PROG_CC
-BTRFS_DETECT_CSTD
 AC_CANONICAL_HOST
 AC_C_CONST
 AC_C_VOLATILE
@@ -48,6 +47,9 @@ AX_CHECK_COMPILE_FLAG([-msha], [HAVE_CFLAG_msha=1], [HAVE_CFLAG_msha=0])
 AC_SUBST([HAVE_CFLAG_msha])
 AC_DEFINE_UNQUOTED([HAVE_CFLAG_msha], [$HAVE_CFLAG_msha], [Compiler supports -msha])
 
+AX_CHECK_COMPILE_FLAG([-std=gnu11], [],
+	[AC_MSG_ERROR([std=gnu11 compiler required])])
+
 AC_SYS_LARGEFILE
 
 AC_PROG_INSTALL

m4/ax_gcc_version.m4

@@ -1,37 +0,0 @@
-dnl @synopsis AX_GCC_VERSION(MAJOR, MINOR, PATCHLEVEL, [ACTION-SUCCESS], [ACTION-FAILURE])
-dnl @summary check whether gcc is at least version MAJOR.MINOR.PATCHLEVEL
-dnl @category InstalledPackages
-dnl
-dnl Check whether we are using gcc and, if so, whether its version
-dnl is at least MAJOR.MINOR.PATCHLEVEL
-dnl
-dnl ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
-dnl success/failure.
-dnl
-dnl @version 2005-05-30
-dnl @license GPLWithACException
-dnl @author Steven G. Johnson <stevenj@alum.mit.edu> and Matteo Frigo.
-AC_DEFUN([AX_GCC_VERSION],
-[
-AC_REQUIRE([AC_PROG_CC])
-AC_CACHE_CHECK(whether we are using gcc $1.$2.$3 or later, ax_cv_gcc_$1_$2_$3,
-[
-ax_cv_gcc_$1_$2_$3=no
-if test "$GCC" = "yes"; then
-dnl The semicolon after "yes" below is to pacify NeXT's syntax-checking cpp.
-AC_EGREP_CPP(yes, [
-#ifdef __GNUC__
-#  if (__GNUC__ > $1) || (__GNUC__ == $1 && __GNUC_MINOR__ > $2) \
-   || (__GNUC__ == $1 && __GNUC_MINOR__ == $2 && __GNUC_PATCHLEVEL__ >= $3)
-     yes;
-#  endif
-#endif
-], [ax_cv_gcc_$1_$2_$3=yes])
-fi
-])
-if test "$ax_cv_gcc_$1_$2_$3" = yes; then
-	m4_default([$4], :)
-else
-	m4_default([$5], :)
-fi
-])

m4/btrfs_detect_cstd.m4

@@ -1,20 +0,0 @@
-dnl We prefer -std=gnu90 but gcc versions prior to 4.5.0 don't support
-dnl it.  AX_CHECK_COMPILE_FLAG is the right way to determine whether a
-dnl particular version of gcc supports a flag, but it requires autoconf
-dnl 2.64.  Since (for now) we still want to support older releases
-dnl that ship with autoconf 2.63, we the also-deprecated AX_GCC_VERSION
-dnl macro there.
-AC_DEFUN([BTRFS_DETECT_CSTD],
-[
- m4_version_prereq([2.64], [
-     AX_CHECK_COMPILE_FLAG([-std=gnu90],
-       [BTRFS_CSTD_FLAGS=-std=gnu90],
-       [BTRFS_CSTD_FLAGS=-std=gnu89])
-   ], [
-     AX_GCC_VERSION([4], [5], [0],
-       [BTRFS_CSTD_FLAGS=-std=gnu90],
-       [BTRFS_CSTD_FLAGS=-std=gnu89])
-   ])
-   AC_SUBST([BTRFS_CSTD_FLAGS])
-]) dnl BTRFS_DETECT_CSTD
-