For *nix, only delete the ppport.h files that we created.

Given that we have the list of places to create ppport.h files easily
accessible, it's easy to delete only the files that we created, and avoid a
find recursing into all of ext/, dist/ and cpan/. This also means that if
anyone drops a module from CPAN into ext/ to build it as part of the core,
any ppport.h it has won't be wiped out by the core. This is one of the few
things that goes wrong, as the rest of the distclean target is very careful
to only delete files that it knows that the build process built.
If miniperl fails the simple Exporter test, print a message and bail out.

Previously the Makefile rule for miniperl would attempt to automatically run
minitest if the simple Exporter test failed. It did this by recursively
calling make in the same directory, which is not a totally safe thing to do
even at the best of times.

However commit cb251201d6951395, necessary to simplify other parts of the
toolchain bootstrapping, has made it easy to trigger an infinite loop of
Makefile recursion if miniperl is unable to run the simple Exporter test.

So, for now, defuse the trap by avoiding the recursive make call. A better
long term solution will be to set up and run minitest by something other than
a Makefile target, such as a shell script. As it's not immediately obvious how
to best integrate this with ./runtests, commit this simpler fix for now.