Setting an environment variable for tests

[Marc Mims]

I recently released a new version of Math::Round::Fair.  Anders Johnson 
provided several in situ tests that we optimized out at compile time 
using Devel::Assert.  They can be turned on by setting an environment 
variable:

    MATH_ROUND_FAIR_DEBUG=1

I wanted the assertions enabled when "make test" runs without having to 
explicitly set the variable in each test.  And I found a way to do that, 
using Module::Install's "preamble":

    ぴreamble "export MATH_ROUND_FAIR_DEBUG=1\n";

That works for Linux systems, but seems to be failing on other 
platforms:

http://www.cpantesters.org/cpan/report/28e28ab0-0825-11e0-bb29-ad544afd17af
http://www.cpantesters.org/cpan/report/68705c5e-086e-11e0-bb29-ad544afd17af


So, how *should* I be doing this?

        -Marc