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>

toolchain/helpers.mk

@@ -193,6 +193,10 @@ check_kernel_headers_version = \
 #
 check_gcc_version = \
 	expected_version="$(strip $2)" ; \
+	if [ -z "$${expected_version}" ]; then \
+		printf "Internal error, gcc version unknown (no GCC_AT_LEAST_X_Y selected)\n"; \
+		exit 1 ; \
+	fi; \
 	real_version=`$(1) --version | sed -r -e '1!d; s/^[^)]+\) ([^[:space:]]+).*/\1/;'` ; \
 	if [[ ! "$${real_version}" =~ ^$${expected_version}\. ]] ; then \
 		printf "Incorrect selection of gcc version: expected %s.x, got %s\n" \