Also check for Python platform-specific include directory

Python can be built to have two separate include directories: one for
platform-independent files and one for platform-specific files.  So
far, this has apparently never mattered for a PL/Python build.  But
with the new multi-arch Python packages in Debian and Ubuntu, this is
becoming the standard configuration on these platforms, so we must
check these directories separately to be able to build there.

Also add a bit of reporting in configure to be able to see better what
is going on with this.