add a pseudo-library "missing", part 3 -- fix the check-configure test
fix makefile so mpir can have its own config options passed to it
fix my makefiles to add gcc option -mmacosx-version-min=10.6

This is an experiment to see if we can deal with the reports that M2
crashes on startup under Mac OS X 10.6.  We compiled under 10.8.
Merge branch 'release-1.6' into release-1.6-mingw