Add a pkg-config file for clients to use

Makefile.am

 SUBDIRS = src
+
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = webrtc-audio-processing.pc

configure.ac

@@ -30,6 +30,7 @@ AC_SUBST([COMMON_CFLAGS])
 AC_SUBST([COMMON_CXXFLAGS])
 
 AC_CONFIG_FILES([
+webrtc-audio-processing.pc
 Makefile
 src/Makefile
 src/common_audio/Makefile

webrtc-audio-processing.pc.in

+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: webrtc-audio-processing
+Description: WebRTC Audio Processing library
+Version: @PACKAGE_VERSION@
+Libs: -L${libdir} -lwebrtc_audio_processing
+Cflags: -I${includedir}/webrtc_audio_processing