Add custom backend flags

[barracuda156]

The bootstrap version does not require clang: https://github.com/axelang/axe-bootstrap/blob/1aa7b02b893ec7470740faec0dcce55a5e8b0667/build_exp.sh#L13-L17

Why is it asked for here?

axe/README.md

Lines 51 to 53 in 7506d82

	### Prerequisite
	[Clang compiler](https://clang.llvm.org/)

P. S. This matters, since clang has inferior cross-platform support to gcc.

[barracuda156]

Okay, apparently requirement in README is wrong:

axe/source/compiler/builds.axe

Lines 425 to 441 in 7506d82

	if command_exists(str("clang")) {
	compiler = str("clang");
	} else {
	if command_exists(str("cc")) {
	compiler = str("cc");
	if !::quiet_mode {
	println "\ncc: Linking.";
	}
	}
	if str_len(compiler) == 0 {
	println "\nYou do not have the LLVM C toolchain installed.";
	println "Install clang from https://releases.llvm.org/ or via your package manager.";
	println "Alternatively, ensure 'cc' is available in your PATH.";
	return false;
	}
	}

But it will be nice to allow setting the compiler explicitly. cc will pick a wrong one.
Also it may not be desirable to use clang even when it exists.

[navid-m]

Clang actually has superior cross-compilation capabilities, is better optimized both in terms of compile-time and runtime, and is more modern overall, that's why it is preferred.

Though, it would probably be a good idea to add custom backend flags. e.g. --backend=gcc, --backend=tcc, --backend=nasm, etc... So I'll leave this open until I get around to that.