make the "alternate" clang build use -O0 optimizations

The alternate clang build currently uses strict compilation = false,
this was requested so that we get as many error messages as possible.

We would also like to have an -O0 build run against the unit tests,
as sometimes -O0 will segfault while -O2 won't, etc. So in this
commit the alternate build uses that option also. I also rename
the environment variables, so that the build matrix is based on
casing out whether "alternate_configuration" is true (not strict_
compilation) as previously, and the alternate_configuration flag
leads to options being exported in the before_install phase.