Nobody cares about BeOS in 2022, and it cannot be built with Meson anyway.
Removing the BeOS backend also allows us to drop the dependency on a C++
compiler and simplify the Autotools build considerably by removing all
the extra CXX targets.
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
