pkg-config: present API version and not package version

This way PKG_MODULE() autoconf test checks for required API
rather than release version (e.g required API could be in all
all of 3.4.5, 3.5.1, 3.6.4 etc.)

With the first "release" of gfapi, the current version (4) will
be frozen.

Version 3 is sufficient for QEMU's requirement. Version 4 is
necessary for Samba VFS

Change-Id: Iada3a4c88b9c67413dcd57004fd8a62926a21b42
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4932
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>