[petsc-dev] RFC: allow setting flags via environment variables

[Sean Farley]

For purposes of building PETSc in a package manager setting, it would
convenient to have a way to turn off the default behavior of not setting
CFLAGS, CXXFLAGS, etc. via environment variables.

The reason I bring this up is because in MacPorts (perhaps others) it is
impossible to know the full set flags until the package is fully
processed. See here for more details:

https://lists.macosforge.org/pipermail/macports-dev/2014-October/028279.html

At the moment, I patch BuildSystem but was wondering if it'd be worth
having a flag (or some other way) to do this.