Allow pcl to be built against system metslib

This patch allows PCL to build against a system installed metslib
where available.  It uses pkgconfig to try to discover metslib.pc,
and when both are available HAVE_METSLIB is set to ON and the
include paths are modified to use metslib's location as discovered
by pkgconfig.  When pkgconfig and/or metslib is not found, cmake
will continue to use the bundled version of metslib as before.
Whitespace cleanup in registration/CMakeLists.txt

While making modifications for metslib, I noticed that there was
some mixed use of tabs and spaces for intentation.  I normalized
them all to 4 spaces per tab and made the indentation of all of
the "set" blocks consistent.
Merge pull request #299 from jpgr87/master

Allow pcl to be built against a system-wide installed metslib