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>
18e12e95ab