Enabled GCC -Wwrite-strings warning for CMake and replaced many 'char's for 'const char's,.

Only one functional change where Transform orientations passed "" to BIF_createTransformOrientation() which could then have the value written into.

source/blender/blenkernel/intern/object.c

@@ -969,7 +969,7 @@ static void *add_obdata_from_type(int type)
 	}
 }
 
-static char *get_obdata_defname(int type)
+static const char *get_obdata_defname(int type)
 {
 	switch (type) {
 	case OB_MESH: return "Mesh";