Fix CMakeLists.txt for non-make generators (#94)

The hack to make __FILE__ relative to the project directory doesn't work with non Make generators, so make it optional instead.
In general, Ninja is a better tool, so it is nice to support

CMakeLists.txt

@@ -24,7 +24,9 @@ project(pulseaudio_modules_bt C)
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 set(CMAKE_C_FLAGS "$ENV{CFLAGS} -O2 -Wall -Wno-builtin-macro-redefined -Wno-unused -fno-common -DFASTPATH")
-set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__FILE__='\"$(subst ${CMAKE_SOURCE_DIR}/src/,,$(abspath $<))\"'")
+if(CMAKE_GENERATOR STREQUAL "Unix Makefiles")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__FILE__='\"$(subst ${CMAKE_SOURCE_DIR}/src/,,$(abspath $<))\"'")
+endif()
 
 option(CODEC_AAC_FDK "LC-AAC support using fdk-aac(-free)" ON)
 option(CODEC_APTX_FF "aptX Classic support using FFmpeg" ON)