Change ${CMAKE_BINARY_DIR} to ${CMAKE_CURRENT_BINARY_DIR}

- if building as a submodule of another repository, installation of pkg-config files can fail because they may not be in the top-level binary directory

- changing ${CMAKE_BINARY_DIR} to ${CMAKE_CURRENT_BINARY_DIR} allow CMake to find the files for installation
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0289704..698133f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -129,7 +129,7 @@
 		"pkg-config/jsoncpp.pc.in"
 		"pkg-config/jsoncpp.pc"
 		@ONLY)
-	INSTALL(FILES "${CMAKE_BINARY_DIR}/pkg-config/jsoncpp.pc"
+	INSTALL(FILES "${CMAKE_CURRENT_BINARY_DIR}/pkg-config/jsoncpp.pc"
 		DESTINATION "${CMAKE_INSTALL_PREFIX}/lib${LIB_SUFFIX}/pkgconfig")
 ENDIF()