avoid 'jobserver unavailable:' warnings

Under GNU make, an interaction between the top-level 'make -j N harness'
and a make invoked by cpan/ExtUtils-Constant/t/Constant.t
causes warnings like:
    warning: jobserver unavailable: using -j1.  Add `+' to parent make rule.

Fix this by stopping MAKEFLAGS being inherited by the children.
The other option, of prefixing all the '$(RUN_TESTS) choose' lines
in Makefile.SH with '+' is, I suspect, not portable.