So that recipes using Module::Build don't need to specify that in their
recipes. Ironically, Module::Build itself doesn't profit from that, because its
build script overrides the default, so we have to set it on the command line
still (or alternatively patch the build file).
Also, unset the variable for tests, otherwise a test would have to be patched
too.
Tests which invoke make or gcc are skipped properly if they are not found.
So these aren't really needed. Also, gcc would need further changes to
actually work on x86 (see the next commit).
This is used as shebang line for installed scripts.
This could be done in a cleaner way by defining a separate Haiku platform file
and overriding some functions there, but for now, this works well enough.
Some tests are broken because it doesn't find a compiler (adding cmd:gcc to
TEST_REQUIRES isn't sufficient it seems, and it would require $secondaryArchSuffix
which isn't available for any arch recipes).
