External Issue: prereqs.rst doesn't note all cases cmake is required

18536, "cassella", "prereqs.rst doesn't note all cases cmake is required", "2021-10-08T05:10:29Z"

doc/rst/usingchapel/prereqs.rst says about cmake only that it's required to build the bundled LLVM. However, it's also required in order to build docs, due to compiler/next/Makefile.help:

CMAKE ?= cmake

And also direct calls to cmake instead of $(CMAKE) in a few places.

(And make docs now reaches rules in that file that use cmake.)

If it's intentional for cmake to now be a prereq for things apart from building teh bundled LLVM, I think the docs should be updated. If not, the makefiles need adjusting.