GCC releases prior to 4.5.0 don't support -std=gnu90 so btrfs-progs won't
build at all on older distros.  We can detect whether the compiler
supports -std=gnu90 and fall back to -std=gnu89 if it doesn't.

AX_CHECK_COMPILE_FLAG is the right way to do this, but it depends on
autoconf 2.64.  AX_GCC_VERSION has been deprecated, so we'll use that
only for earlier autoconf versions so we can drop it when we drop
support for older autoconf releases.

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