rpm: added individual override variables for opensync building

rpm: added individual override variables for opensync building

If you set ADD_TO_PKG_CONFIG_PATH, it will be prepended to
PKG_CONFIG_PATH before building anything.

If you set OSYNC4X_PKG_CONFIG_PATH, it will be prepended to
the original PKG_CONFIG_PATH before building the 0.4x plugin.

Same for OSYNC2X_PKG_CONFIG_PATH with the 0.2x plugin.

And set OSYNCBOTH_PKG_CONFIG_PATH for building the desktop.

If you set any of the OSYNC* variables, set them all, since they
are all all set in order, and override each other.  If you don't
set them all, 0.2x settings will be used for the 0.4x plugin, etc.

In other words, if you have one setting for the whole build, use
ADD_TO_PKG_CONFIG_PATH.  If you need individual settings for the
opensync plugins and desktop, use *all* of the OSYNC* variables.
It is safe to use both ADD* and OSYNC* variables.  ADD is used
first, then the OSYNC variables as needed.

If you need help, see the barry.spec file for the simple logic.