Permit multiple packing list files to be listed in PLIST.

[jhb]

Summary

This permits a port to use separate packing lists for optional components
instead of depending on PLIST_SUB.

Test Plan

• used to test a split-out pkg-plist file for devel/freebsd-gcc6

Other possible candidates include devel/binutils, base/binutils, and
base/gcc.

[jhb]

There may be better ways of doing this, this was just a first cut. You can see an example of this being used in the child revision in the stack (D22451)

[bapt]

I was thinking about adding a @include in plist to allow this, but this is simpler I do like it.

[jhb]

I think this probably warrants an exp-run before committing?

[adamw]

In D22450#491430, @jhb wrote:

I think this probably warrants an exp-run before committing?

There are only two ports with spaces in ${PLIST}, and both of those are whitespace errors.

[adamw@apnoea /usr/ports] ag '^PLIST=.* ' 
devel/libpeas/Makefile
35:PLIST=          ${.CURDIR}/pkg-plist

devel/luafilesystem-51/Makefile
11:PLIST=          ${MASTERDIR}/pkg-plist

Unless I'm missing something, if ports in your own sample build fine, I don't see where any others would fail.

[jhb]

Ok, I did do some test builds of a "normal" port when testing this as well (just using default PLIST), and I do think it's true that if this broke it would break everything.