Use OBJS_SRCS_FILTER to control setting OBJS from SRCS

Some makefiles do reachover builds.
In some cases it is convenient to list subdirs of the distribution
in SRCS.

It is not very convenient, or always even desirable to have corresponding
subdirs in .OBJDIR, so OBJS_SRCS_FILTER allows the makefile to choose.
The default value 'R' matches existing practice.

But a makefile can set OBJS_SRCS_FILTER= T (the R gets added by
bsd.init.mk) to avoid the need for subdirs in .OBJDIR

Differential Revision: https://reviews.freebsd.org/D12218
Reviewed by: bdrewery