METIS easyblock: No support for preconfigopts

[klust]

The METIS easyblock currently does not honour preconfigopts for METIS version 5 and above (didn't check for earlier versions). I think this could be easily fixed by modifying line 62 of metis.py to
cmd = "%s make %s config prefix=%s" % (self.cfg['preconfigopts'], self.cfg['configopts'], self.installdir)

The advantage of this would be that rather than using a patch file to set the desired integer and floating point data types in include/metis.h, this could then be done using sed through preconfigopts, e.g.,
preconfigopts = 'sed --in-place=".orig" '
preconfigopts += ' 's/#define IDXTYPEWIDTH./#define IDXTYPEWIDTH 32/; '
preconfigopts += ' s/#define REALTYPEWIDTH./#define REALTYPEWIDTH 64/' include/metis.h && '

This would make it much easier to compile METIS for various choices of the data types as it would no longer be needed to generate the (currently very trivial) patch file.