Makefile: fix build for directories with spaces

When full path of a source directory has spaces in it, that makes
shell and Make split the path, so special treatment is
necessary. Additionally, @F doesn't honour the escaping, so has to be
avoided.

Reported-by: Roman Faizullin <roman@faizullin.info>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>