devel/llvm90 requires math/z3 first; building math/z3 requires a c++ toolchain be in place

[Mark Millard]

Building math/z3 involves:

# grep compiler /usr/ports/math/z3/Makefile
USES=		compiler:c++11-lang python:2.7,build

But devel/llvm90 requires math/z3 to have been built before
devel/llvm90 is built:

# pkg info -d llvm90
llvm90-9.0.0.r1:
	libxml2-2.9.9
	z3-4.8.5_1
	python36-3.6.9
	perl5-5.28.2
	libedit-3.1.20190324,1
# pkg info -B llvm90
llvm90-9.0.0.r1:
	libpython3.6m.so.1.0
	libedit.so.0
	libz3.so.0
	libxml2.so.2


Hopefully this cycle can be avoided for system
clang to eventually have progressed to clang 9.
(I do not know the details.)

For architectures still at gcc/g++ 4.2.1, some
alternate c++ tool chain needs to be used to
build libz3.so but the result needs to be
compatible with llvm90 later using the libz3.so's
content. (I do not know the details.)

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)