What's the least required in base to be functional?

Apologies. That may not have been the best choice of titles.
What I'm trying to determine, is what is the very least I will
require in base, to actually build a userland build environment.
NOTE; this all concerns -CURRENT (recent 11).
Point being, while I recognize that clang/llvm is the default on
10+. I have been building/installing world/kernel with

make.conf(5)
WITHOUT_CLANG=true
FAVORITE_COMPILER=gcc

src.conf(5)
WITHOUT_CLANG=true

on RELENG_8, and RELENG_9, and 11 (as of 1 mos ago)
Everything worked as anticipated. But a recent (5 days ago)
build/install on -CURRENT. Followed by a make delete-old
_seemed_ to have an adverse affect. More specifically;
having used the above declarations always resulted in the
make delete-old removing clang from base. Which was fine. As
I had intended to experiment with the different versions of
lang/clang, and devel/llvm, via installing from ports. But my
recent attempt using the above method, resulted in my being
unable to build many ports. x11/* mostly. I ran into problems
with "xmmintrin.h" not being found. Or other problems, where
declarations were not supported in gcc(4.8,4.9, or 5). So what
exactly *must* be installed in base to allow for a more
*granular* approach to testing/building?
Used to be IIRC, fmake, or bmake. But that's likely a pretty
dated recollection.

Thank you for all your time, and consideration.

--Chris