CMake: Use BUILD_SHARED_LIBS intead of ENABLED_SHARED, ENABLE_STATIC

[dofuuz]

Fix #438
#403

• Use BUILD_SHARED_LIBS intead of ENABLED_SHARED, ENABLE_STATIC
• Builds either shared or static at a time. Not both at once.

Building shared/static one by one and using BUILD_SHARED_LIBS is more standard way to using CMake.
https://cgold.readthedocs.io/en/latest/tutorials/libraries/static-shared.html

This going to fix shared/static libpng/zlib problem.

It may break existing automated builds.

[kornelski]

Wow, I would not have guessed that a variable name can have a side effect.

[kornelski]

Done in 38869c2