BUILD_DEFS and X11_TYPE and PKG_SYSCONFBASE and VAR and ...

["Jeremy C. Reed" <reed%reedmedia.net@localhost>]

I use X11_TYPE=modular and I noticed there was no indication in my 
installed packages metadata.

Okay to have BUILD_DEFS include X11_TYPE for X11 packages?
Like have mk/x11.buildlink3.mk and x11/xproto/buildlink3.mk define:
BUILD_DEFS+=            PKG_SYSCONFBASE
(hopefully not repeated as I have not tested yet)
similar to security/openssl/buildlink3.mk which has:
BUILD_DEFS+=       SSLBASE

On a related note, I see many makefiles define:
BUILD_DEFS+=    PAPERSIZE
or
BUILD_DEFS+=                VARBASE
or
BUILD_DEFS+=           PKG_SYSCONFBASE

Shouldn't the mk infrastructure add these to BUILD_DEFS if used or 
defined? Or is placing in the individual package's makefiles correct?

I don't see BUILD_DEFS documented.