[Bug #599248] ext module generation problem

If you have source files srcdir1/foo.c and srcdir2/foo.c, the
temporary .o for both files is written to build/temp.<platform>/foo.o.
This patch sets strip_dir to false for both calls to object_filename,
so now the object files are written to temp.<platform>/srcdir1/foo.o
and .../srcdir2/foo.o.

2.2 bugfix candidate