This should be an equivalent to what system GCC of CentOS Stream 10
already has [1].
Because gcc-toolset-15-gcc will be scl-less and I have no prototype
to work with, let's take this as a first draft for now. Definitely
using the WITH_SCL variable is confusing and an obvious candidate
for an improvement...
Related: RHEL-92605
[1] 5780c4d2e3
