btrfs-progs: Specify C standard to gnu90 explicitly

btrfs-progs: Specify C standard to gnu90 explicitly

Different C compilers have different default language standard.
This sometimes causes problem on different system.

For distribution like CentOS/RHEL7, its gcc is still 4.8 and will report
error for c90 style declaration, while most developers are using newer
gcc which will just ignore it.
This makes us hard to detect such language standard problem.

This patch will specify standard to gnu90 explicitly to avoid such problem.
Gnu90 is a good mix of c90 while provide a lot of useful gnu extension,
and is supported by all modern gcc and clang.

Reported-by: Marco Lorenzo Crociani <marcoc@prismatelecomtesting.com>
Signed-off-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Signed-off-by: David Sterba <dsterba@suse.com>