This is an attempt to fix the broken situation we've been in where
automake links libcairo.la with c++ because it might potentially maybe
include C++ files.
Those potential files only exist in Chris' throwaway backends (skia, qt)
and the BeOS backend, so for 99.99% of cases, these backends are not
needed and linking with c++ is overkill. Also, no one wants to have
libcairo.so link to libstdc++.
This patch fixes that in mutliple steps:
1) Add build infrastructure to distinguish between C and C++ backends.
This is done by allowing to specify backend_sources as well as
backend_cxx_sources variables in Makefile.sources.
2) Optionally build a libcairo_cxx.la noinst library
This intermediate library is built for C++ backends only and therefor
linked using c++. It is then linked into the final libcairo.la. This
does not require c++, so the linking of libcairo.la is done with cc.
This also works around various weirdnesses that the current build system
exposes, where it assumes cisms when in fact using c++ semantics, like
not detecting c++ properly or:
https://bugzilla.redhat.com/show_bug.cgi?id=606523
