Correct typo which made distclean target malfunction

Probably typed _DIRECTORY instead of _DIR since there were other
variables nearby with that suffix.
Allow user to specify another version of CMake

On CentOS, cmake28 must be used rather than the standard cmake
Don't remove original files from in-source trees

If we are building in-source, some files are not copied because they
would end up being the same, e.g. GNUmakefile.
Provide instr. for installing deps. on CentOS

Currently these doesn't include packages for SuperLU and DUNE since they
are not part of EPEL and must be provided on their own.
Use TinyXML instead of LibXML2

The code only uses TinyXML, so the list of dependencies was wrong,
something that was covered by the bundled version of TinyXML.
Look for CMake 2.8 if available on system

We require at least this version, and if there is a newer, it will
probably break due to lack of backward compatibility anyway.
Prevent conflict between older Boost and newer CMake

The probe for Boost was changed in CMake 2.8; if the system has
installed configuration files tailored for an older CMake, the
library paths are not setup correctly. This patch disables using
those configuration files; just use the library directly instead.
Allow CMake program to be specified with env. var.

If you are using a custom CMake, you should be able to set the
environment variable "CMAKE" and have the configure script pick up
the default from there so that you don't have to specify the option
every time.
Probe various candidates for system release info

Not all Linux distributions is LSB-compliant, notably RHEL 6, so we
should make an effort to check various known other files in order to
identify the distribution.
Merge pull request #145 from rolk/0145_centos

Make opm-core tests compile on CentOS 6.3