Remove PROJECTM_EXPORT from PCM class methods

[revmischa]

PCM is an internal class with the C API (projectm_pcm_*) providing the public interface. Exporting these methods caused symbols to leak into the shared library even when ENABLE_CXX_INTERFACE was off.

Verified with nm -D that the C++ PCM class symbols are no longer exported while the C API functions remain available.

Fixes #937

[kblaschke]

I was already working on that issue, see my early WIP branch: https://github.com/kblaschke/projectm/commits/fix-pcm-exports/

There are actually exports missing instead of being too many. These exports are required for the ENABLE_CXX_INTERFACE option, otherwise libprojectM (static or shared) won't link to apps using the C++ interface due to unresolved symbols. The actual fix requires a new export header for the C++ interface, so these export macros can be toggled separately. So there's the existing PROJECTM_EXPORT macro for the default C API functions, and I'll add a new one, PROJECTM_CXX_EXPORT, for the C++ interface.

[kblaschke]

Properly fixed the whole ordeal, which was way more complex than I thought due to how Windows declares exports/imports and some other things which were wrong or missing in the CMake files. Here's what I've done:

• Always build the project with "hidden" default visibility, e.g. no exported symbols except those marked as such.
• Added another export header for the C++ interface, so the exports can be enabled separately.
• Added the new C++ export macro 'PROJECTM_CXX_EXPORT` to all required classes and static functions (Logging).
• Fixed how those export macros are set or used within the code base.
• Added a small test executable that can be built to verify the required classes are properly exported and thus can be linked, even with a shared projectM library.
• Added the C++ API checks as an additional build step on desktop platforms. Android and Emscripten are basically Linux builds, so I don't test it in those.

Depending on how projectm-eval has been built, its symbols might end up being visible in nm's output. Besides that, only the C API and, if the C++ interface is enabled, the specifically exported classes and members are now visible externally.