add cmake Find/Use for system libraries

[rainbreak]

Will allow us to more easily reason with system dependencies, with the aim of allowing full static linking.

[rainbreak]

The only problem with this one is in UseZLIB:

if (ZLIB_FOUND)
        target_include_directories(${TARGET} SYSTEM PUBLIC ${ZLIB_INCLUDE_DIRS})
        # target_link_libraries(${TARGET} ${ZLIB_LIBRARIES})
        target_link_libraries(${TARGET} z)

The commented out target_link_lib line is what I think it should be, but the actual line is what worked for me (Alpine Linux).

The problem with the commented line is that it tries to link -lzlib which isn't valid. The uncommented line gives us -lz as desired.

In principle this should all be handled in FindZLIB, which is included with cmake and has set(ZLIB_NAMES z zlib zdll zlib1).

[bobsummerwill]

RE: zlib. Right - thanks for the info.

Well, it can stick as it is for now, because we need to get better conditionals higher up before we even hit this issue in the code which is currently committed. Maybe have a Google around for this one, which I would imagine other people have hit too.

"CMake zlib library name inconsistent" or whatever?

Or we just hardcode it as you have, with some comment on why.