automake: Fix rtaudio.pc prefix variable

CMakeLists.txt

@@ -257,6 +257,7 @@ message(STATUS "Compiling with support for: ${apilist}")
 # PkgConfig file
 string(REPLACE ";" " " req "${PKGCONFIG_REQUIRES}")
 string(REPLACE ";" " " api "${API_DEFS}")
+set(prefix ${CMAKE_INSTALL_PREFIX})
 configure_file("rtaudio.pc.in" "rtaudio.pc" @ONLY)
 
 # Add install rule.

rtaudio.pc.in

@@ -1,4 +1,4 @@
-prefix=@CMAKE_INSTALL_PREFIX@
+prefix=@prefix@
 exec_prefix=${prefix}
 libdir=${exec_prefix}/lib
 includedir=${prefix}/include/rtaudio