Allow relative pathnames in SRCS, so as to enable building software
which includes more than one file with the same name, in different
directories.

For example, setting:

SRCS+= foo/foo.c bar/foo.c baz/foo.c

will now create separate objdirs 'foo', 'bar' and 'baz' for each of the
sources in the list, and use those objdirs for the corresponding object
files.

Reviewed by: brooks, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D1984