Simplify library dependency flags.

We have a confusing array of nearly-identical CFLAGS/LIBS definitions in
configure.ac.  Time to simplify.  Instead let's just have one definition
that includes all the libraries provided by Evolution-Data-Server (incl.
Camel).  That, in combination with GNOME_PLATFORM, gives us most of what
we need for compliation and linking, and we can sprinkle definitions for
additional library dependencies in Makefile.am's as needed.