Use relative path to set cmake module path

This has actually no effect on a "normal" build, but allows to use
libgit2 as a part of a larger project via CMake's ADD_SUBDIRECTORY()

Closes #2087

CMakeLists.txt

@@ -15,7 +15,7 @@ PROJECT(libgit2 C)
 CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
 
 # Add find modules to the path
-SET(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
+SET(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_LIST_DIR}/cmake/Modules/")
 
 # Build options
 #