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
Differential Revision: https://reviews.freebsd.org/D1984
4c1f391878