Use pkg-config trick for Carla on macOS only if installed

cmake/modules/CarlaPkgConfig.cmake

@@ -1,9 +1,10 @@
 # Makes carla submodule available to PKG_CHECK_MODULES
 SET(CARLA_SUBMODULE_SOURCE ${CMAKE_SOURCE_DIR}/plugins/carlabase/carla)
 SET(CARLA_PKGCONFIG_FILE ${CARLA_SUBMODULE_SOURCE}/data/carla-standalone.pc)
+SET(CARLA_LIB_FILE "/Applications/Carla.app/Contents/MacOS/libcarla_standalone2.dylib")
 
 # Look for carla-standalone pkg-config file
-IF(EXISTS ${CARLA_PKGCONFIG_FILE})
+IF(EXISTS ${CARLA_PKGCONFIG_FILE} AND EXISTS ${CARLA_LIB_FILE})
 	SET(ENV{PKG_CONFIG_PATH} "$ENV{PKG_CONFIG_PATH}:${CMAKE_BINARY_DIR}")
 	FILE(READ ${CARLA_PKGCONFIG_FILE} CARLA_PKGCONFIG_TEXT)
 	# add source/includes to include location