Dropped CPPFLAGS from generated pkg-config

CPPFLAGS are needed to compile rtaudio.
these are not the same flags as needed for compiling an application using rtaudio.
sometimes this is even harmful.

Closes: https://github.com/thestk/rtaudio/issues/103

Signed-off-by: IOhannes m zmölnig <zmoelnig@umlautQ.umlaeute.mur.at>

rtaudio.pc.in

@@ -9,4 +9,4 @@ Version: @PACKAGE_VERSION@
 Requires: @req@ 
 Libs: -L${libdir} -lrtaudio
 Libs.private: -lpthread
-Cflags: -pthread -I${includedir} @CPPFLAGS@
+Cflags: -pthread -I${includedir}