make: improve diagnostic messages when the prebuilt tcc executable does not work/is missing

cmd/tools/detect_tcc.v

@@ -0,0 +1,15 @@
+fn main() {
+	$if tinyc {
+		println('Your `tcc` is working. Good - it is much faster at compiling C source code.')
+		exit(0)
+	}
+
+	println('
+NB: `tcc` was not used, so unless you install it yourself, your backend
+C compiler will be `cc`, which is usually either `clang`, `gcc` or `msvc`.
+
+These C compilers, are several times slower at compiling C source code,
+compared to `tcc`. They do produce more optimised executables, but that
+is done at the cost of compilation speed.
+')
+}