Let user supply paths for installed packages.

[gnperdue]

Let the user supply required paths for packages (e.g., $ROOTSYS, etc.) so we can take advantage of pre-installed software when it is available!

[mnirkko]

I do this for the ROOT, GSL and PYTHIA versions we are using at T2K. Currently, I find & replace a few lines in build_support.sh to achieve this.

Don't forget that it's a big difference whether the users install software (e.g. ROOT) with just "make" or if they follow it up with "make install"! I've found that my libraries can be in either $ROOTSYS/lib/root or $ROOTSYS/root/lib (or both), which is quite annoying. :)

Probably best to automatically check if software is already installed, and where. Maybe with "which root-config"?