maintcheck: fix maintainer-check failures, both real and spurious

maintcheck: fix maintainer-check failures, both real and spurious

* tests/amhello-binpkg.test: Use "$MAKE", not bare "make".
* Makefile.am (sc_perl_local): Also allow perl special variable
`$~' to be localized.  And be slightly laxer in the regexp, to
allow for usages like "local $_ = $foo;".
(sc_tests_overriding_macros_on_cmdline): Also allow for command
line overriding of the `DISABLE_HARD_ERRORS' make variable.  Try
to avoid false positives for usages like "$MAKE || st=$?".