Add path for PhysX default installation on 64-bit Windows systems.
Don't build tools for which the script won't prepare the system.

Our current simple instructions do not include any mention of, nor does the current devlibs.zip contain, Qt5 (required for building Tools) or PyGTK (required to build resource.dat).  This removes those projects by default for people using this script.
Check if CMake is accessible before trying to invoke it.

The default option in CMake's installer is to not install itself into the path.  Our directions on http://h-uru.github.io/Plasma/ remind the user to select the correct option, but they may overlook this, and be faced with an error running this script.  Adding a specific message here during this case will help guide them to self-fixing the problem.
Merge pull request #453 from Deledrius/ezbuild

Simplify automated build process.