Rework the CUnit detection infrastructure as you can't locate include files...

Rework the CUnit detection infrastructure as you can't locate include files based on an absolute path. This is because autoconf and the compiler have differing ideas of the current paths (this totally breaks the Win32 build since you have the extra mapping between UNIX-type MingW paths and Windows-type paths). Hopefully this should not affect existing users at all - if so, please post a report on -devel. I'll try running this on MingW at a later date.

I've also enforced "make check" to run the unit tests, rather than "make test". This is a fallout from the early days when strk committed the regression test suite to run using "make test" by accident which appears to have stuck :(
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git-svn-id: http://svn.osgeo.org/postgis/trunk@3492 b70326c6-7e19-0410-871a-916f4a2858ee