tests: cc-compat.sh: fixed bug when all tested compilers aren’t present

tests/cc-compat.sh

@@ -12,7 +12,7 @@ set -e
 if ! ls GNUmakefile >/dev/null 2>&1
 then
 	printf '%s: Run this script in the root of the project.\n' "$0" 1>&2 
-	exit 1
+	exit 64 # sysexits.h(3) EX_USAGE
 fi
 
 make clean
@@ -24,6 +24,10 @@ for CC in cc\
 	'zig cc'
 do
 	export CC
+
+	command -v "$(printf '%s\n' "$CC" | cut -d ' ' -f1)" >/dev/null 2>&1 \
+		|| continue
+
 	printf '%s: %s: Testing build.\n' "$0" "$CC"
 
 	make CC="$CC" && printf '%s: Build successful.\n' "$0"