reference the pkgconfig file differently in cmake

- reference it in the directory next to the CMakeLists file not in
  the current working directory
- this allows this project to be included (via add_subdirectory())
  as a sub-project in other CMake projects

CMakeLists.txt

@@ -14,7 +14,7 @@ add_library(GPMF_PARSER_LIB ${LIB_SOURCES})
 set_target_properties(GPMF_PARSER_LIB PROPERTIES OUTPUT_NAME "${PROJECT_NAME}")
 
 set(PC_LINK_FLAGS "-l${PROJECT_NAME}")
-configure_file("${CMAKE_SOURCE_DIR}/${PROJECT_NAME}.pc.in" "${CMAKE_BINARY_DIR}/${PROJECT_NAME}.pc" @ONLY)
+configure_file("${CMAKE_CURRENT_SOURCE_DIR}/${PROJECT_NAME}.pc.in" "${CMAKE_BINARY_DIR}/${PROJECT_NAME}.pc" @ONLY)
 
 install(TARGETS GPMF_PARSER_BIN DESTINATION "bin")
 install(TARGETS GPMF_PARSER_LIB DESTINATION "lib")