Libvpx is built with configure script and make.
Unfortunate but the configure script is very dense and I'm
not sure if it's worth translating into CMake, since it
apparently does CPU detection for optimizations.
Remove SRB2_CONFIG_SYSTEM_LIBRARIES condition. This is
because I am lazy and don't want to write
a FindACSVM.cmake. I also don't think it will be common
for it to be installed at the system level.
Currently this breaks some parts of the ACS functionality ... before I made a handful of small extensions to ACSVM's slightly-limited C API myself, and I didn't get them into the actual repo yet.
But now that we've moved to actual C++ compiling, I will likely just flat-out rewrite the SRB2 side of the code in C++ so it can use the library directly.
In environments without zlib installed, png would fail to build since
it can't find zlib.h. The zlib build's generated public include dir
needs zlib.h to work.
openmpt fails to build because it can't find Rpcrt4, which is not a
necessary link under mingw, but exists in some mingw-w64 toolchains.
It is only needed for MSVC.
(cherry picked from commit fcf69001ada08077f82d3fc8510dcd24b4433e64)
