Install steps check for PREFIX_GCC4TI before running

[TC01]

This fixes the potential issue discussed in #ti that if someone tries to run the install scripts directly, there's no check that PREFIX_GCC4TI is defined and some commands (like rm) are unsafe.

I made these changes and tested them some weeks ago, but forgot to push them until just now.

[debrouxl]

I had integrated a version of your patch with one fewer empty line, which created a conflict with this PR. As it stands, it can be closed :)