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 #462

Closes #491

Closes #512

Fixes #509

Fixes #506