cmake: build zlib manually in internal build
Separated from !1854 (merged).
The zlib CMakeLists.txt has a lot of errant effects on the build that can't be easily worked around, particularly with CTest. It's easier to just build it using the downloaded sources with our own script.
- Additionally makes openmpt not build with debug symbols because it blows up in size
- Additionally copies windows dlls to build/bin when building internal shared libraries for convenience
- Additionally adds /build to gitignore
- Additionally disables in-source builds
Edited by Eidolon