toolchain/external: ensure gcc version is known

Currently, when a preconfigured prebuilt toolchain forgets to specify
its gcc version, the error message is a bit misleading, like:

    Incorrect selection of gcc version: expected .x, got 4.9.2

Add a an explicit check for the gcc version being set, that reports a
better error message.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>