cmake: Fix png and openmpt builds in clean envs

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.