Tweak include/lib dir detection in ffi.py

Joint work with @carlosmn

pygit2/ffi.py

@@ -112,9 +112,9 @@ with codecs.open(decl_path, 'r', 'utf-8') as header:
 
 # if LIBGIT2 exists, set build and link against that version
 libgit2_path = getenv('LIBGIT2')
-include_dirs = []
-library_dirs = []
-if libgit2_path:
+if not libgit2_path:
+    libgit2_path = '/usr/local'
+
 include_dirs = [path.join(libgit2_path, 'include')]
 library_dirs = [path.join(libgit2_path, 'lib')]