Avoid calling libtool to link every single test case, by building just one
binary from all the sources.
This binary is then given the task of choosing tests to run (based on user
selection and individual test requirement), forking each test into its own
process and accumulating the results.
By introducing $(top_srcdir)/cairo-version.h I tried to make it
unnecessary to rebuild the entire library when the version changes.
That comes handy around releases and when bisecting.
However this was not working as the version info was also ending
up in config.h and hence forcing a full rebuild upon change.
I now undefine the PACKAGE_VERSION* macros that autoconf defines,
so config.h does not change when cairo-version.h changes.
