cmake/platform/win32.cmake: Remove common CFLAGS from here.

Moved them to win32-gcc.cmake and win32-msvc.cmake.
Wrap the PDB filename settings in IF(MSVC).

libmodutils: Added PDB filename settings.
[extlib] Added CMakeLists.txt files.

Once again, I forgot to add files to the repository.

These should have been added in commit 7f7d4fe3f1f04418ec74bc5a7584691674443803.
([extlib] Install BASS and associated DLLs.)