changes to allow embedding

changes several usages of CMAKE_SOURCE_DIR to CMAKE_CURRENT_SOURCE_DIR which allow embedding the assimp build in a larger CMake project

... as an alternative to dependency-hell

CMakeLists.txt

@@ -23,7 +23,7 @@ add_definitions( -DOPENDDL_NO_USE_CPP11 )
 # Get the current working branch
 execute_process(
     COMMAND git rev-parse --abbrev-ref HEAD
-    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
     OUTPUT_VARIABLE GIT_BRANCH
     OUTPUT_STRIP_TRAILING_WHITESPACE
 )
@@ -31,7 +31,7 @@ execute_process(
 # Get the latest abbreviated commit hash of the working branch
 execute_process(
     COMMAND git log -1 --format=%h
-    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
     OUTPUT_VARIABLE GIT_COMMIT_HASH
     OUTPUT_STRIP_TRAILING_WHITESPACE
 )