Do not build ZynAddSubFX on OS X

Do not build the ZynAddSubFX plugin on OS X until a proper replacement
for unnamed semaphores (which for whatever reason are not supported on
OS X) has been implemented. Using the win32 code (QSystemSemaphore)
could be an option but needs testing.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
(cherry picked from commit fd9d2b0e61)

plugins/zynaddsubfx/CMakeLists.txt

@@ -1,3 +1,5 @@
+IF(NOT LMMS_BUILD_APPLE)
+
 IF(LMMS_HAVE_FFTW3F)
 
 INCLUDE(BuildPlugin)
@@ -137,3 +139,5 @@ ADD_CUSTOM_TARGET(libfltk COMMAND mkdir -p ${CMAKE_CURRENT_BINARY_DIR}/fltk && c
 ADD_DEPENDENCIES(remote_zynaddsubfx libfltk)
 ENDIF(LMMS_HAVE_FFTW3F)
 
+ENDIF(NOT LMMS_BUILD_APPLE)
+