A lot easier than handle sf2 vs sf3 separately.
Closes #717

Often ends up in wrong dir, and did not prove useful in the end
Fixes #678

This finishes the BSD port, hurray!

Closes #533

This adds back audio_decoder library as module as well.
Likely breaks bridges build, to be handled soon.

The top-level Makefile has three library targets which invoke submakes
in source/theme. Since the submake targets share object files, and make
isn't aware of dependencies between different make processes, a parallel
build can try to build the same object file more than once. This results
in failures like this, where two makes are trying to build the same
object:

Linking carlastyle.so
../../build/theme/Release/CarlaStyle.cpp.qt5.o: file not recognized: File truncated
collect2: error: ld returned 1 exit status

Work around this by having all the theme*.a targets depend on theme.

Without this, a parallel build will sometimes fail in source/interposer with:
g++: error: ../../build/modules/Release/juce_core.a: No such file or directory

Closes #512