tools/test.sh: Without a TARGET, use $CC if defined

cc can be absent in Gentoo to make sure the right compiler is picked,
for example when clang is preferred or when cross-compiling.

tools/test.sh

@@ -79,8 +79,8 @@ init() {
 			cc="cc"
 			;;
 		*)
-			cc="cc -no-pie"
-			testcc "$cc" || cc="cc"
+			cc="${CC:-cc} -no-pie"
+			testcc "$cc" || cc="${CC:-cc}"
 			;;
 		esac
 		TARGET=`$bin -t?`