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

 #!/bin/sh

 # Needed for systems without gettext

 $* -xc -o /dev/null - > /dev/null 2>&1 << EOF

 #include <libintl.h>

 int main()

 {

 	gettext("");

 	return 0;

 }

 EOF

 if [ ! "$?" -eq "0"  ]; then

 	echo -DKBUILD_NO_NLS;

 fi