btrfs-progs: autogen: Make build success in CentOS 6 and 7

btrfs-progs: autogen: Make build success in CentOS 6 and 7

btrfs-progs build failed in CentOS 6 and 7:
 #./autogen.sh
 ...
 configure.ac:131: error: possibly undefined macro: PKG_CHECK_VAR
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
 ...

Seems PKG_CHECK_VAR is new in pkgconfig 0.28 (24-Jan-2013):
http://redmine.audacious-media-player.org/boards/1/topics/736

And the max available version for CentOS 7 in yum-repo and
rpmfind.net is: pkgconfig-0.27.1-4.el7
http://rpmfind.net/linux/rpm2html/search.php?query=pkgconfig&submit=Search+...&system=centos&arch=

I updated my pkgconfig to 0.30, but still failed at above error.
(Maybe it is my setting problem)

To make user in centos 6 and 7 building btrfs-progs without
more changes, we can avoid using PKG_CHECK_VAR in following
way found in:
https://github.com/audacious-media-player/audacious-plugins/commit/f95ab6f939ecf0d9232b3165f9241d2ea9676b9e

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>