Check for the CC environment variable when cross compiling

Several people have a habit of setting the CC environment variable when cross compiling Qtopia Core. This of course causes problems because our build system tries to make some native binaries (qmake, moc, uic, rcc) and if the above variable is set, these binaries are cross compiled instead of built for the host. The result is the following error:

.obj/release-shared-emb-arm/adler32.o: could not read symbols: File in a wrong format.

The configure script should check for this variable and exit with an error if this is set.