pkg-config: make the version in gluster-api.pc backwards compatible

Once we released gluster-api.pc with version=4. Since then we undid the
library versioning and replaced it with symbol-versioning. The current
libgfapi.so has version 0, but the symbols have the version from the
main package at the time they were added.

Because other packages (like samba) use the pkg-config version, we can
not drop it, or decrease the version easily. The simplest solution is to
keep the version=4 and add sub-digits for the actual package/symbol
versions. In future, the version in gluster-api.pc will be like 4.3.x.y.

Change-Id: If359c8de5b847a7e818199a7246636113837da2d
BUG: 1166232
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/9154
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>