kbuild: improve scripts/gcc-version.sh output a bit when called without args

Currently, if you call scripts/gcc-version.sh without arguments it will
generate this output :

$ sh scripts/gcc-version.sh
scripts/gcc-version.sh: line 12: [: =: unary operator expected
scripts/gcc-version.sh: line 16: -E: command not found
scripts/gcc-version.sh: line 17: -E: command not found
0000

Not too pretty. I believe this is an improvement :

$ sh scripts/gcc-version.sh
Error: No compiler specified.
Usage:
        scripts/gcc-version.sh <gcc-command>
Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/gcc-version.sh

@@ -9,10 +9,16 @@
 # gcc-2.95.3, `030301' for gcc-3.3.1, etc.
 #
 
-if [ $1 = "-p" ] ; then with_patchlevel=1; shift; fi
+if [[ $1 = "-p" ]] ; then with_patchlevel=1; shift; fi
 
 compiler="$*"
 
+if [ ${#compiler} -eq 0 ]; then
+	echo "Error: No compiler specified."
+	echo -e "Usage:\n\t$0 <gcc-command>"
+	exit 1
+fi
+
 MAJOR=$(echo __GNUC__ | $compiler -E -xc - | tail -n 1)
 MINOR=$(echo __GNUC_MINOR__ | $compiler -E -xc - | tail -n 1)
 if [ "x$with_patchlevel" != "x" ] ; then