Add SINGLE_PROCESS build flag.

Not sure this is strictly necessary, as the SINGLE_PROCESS flag is
generally used for Nexus-specific code, whereas projectM only speaks GL.
diff --git a/src/libprojectM/CMakeLists.txt b/src/libprojectM/CMakeLists.txt
index 02455b4..7680ce8 100644
--- a/src/libprojectM/CMakeLists.txt
+++ b/src/libprojectM/CMakeLists.txt
@@ -43,6 +43,8 @@
   CACHE PATH "The subdirectory relative to the install prefix where libraries will be installed (default is /lib${LIB_SUFFIX})"
   FORCE)
 
+ADD_DEFINITIONS(-DSINGLE_PROCESS)
+
 SET(projectM_SOURCES projectM.cpp PCM.cpp Preset.cpp fftsg.cpp KeyHandler.cpp
 timer.cpp wipemalloc.cpp PresetLoader.cpp  PresetChooser.cpp PipelineMerger.cpp ConfigFile.cpp  TimeKeeper.cpp PresetFactory.cpp PresetFactoryManager.cpp)