scripts: check for, and warn about an unset CT_PREFIX_DIR

The user shall provide a directory to install the toolchain into.
If he/she does not, this is an error, and shall be detected properly,
rather than relying on failure down the road.

Thanks to "Pedro I. Sanchez" <psanchez@colcan.ca> for pointing out
the issue:
http://sourceware.org/ml/crossgcc/2009-12/msg00011.html

 # Overall toolchain configuration: paths, jobs, etc...

 menu "Paths and misc options"

 source "config/global/ct-behave.in"

 source "config/global/paths.in"

 source "config/global/download.in"

 source "config/global/extract.in"

 source "config/global/build-behave.in"

 source "config/global/logging.in"

 endmenu