crt0.h not found on alpine linux (MUSL) about bx

[bkaradzic]

BX_CRT_MUSL=1 not EX_CRT_MUSL=1

[bkaradzic]

MUSL does not have way to get identified so you have to add that define:

From:
https://www.openwall.com/lists/musl/2013/02/08/9

This kind of macro intentionally isn't provided, because its presence
would result in the vast majority of projects treating bug reports of
the form "your program does not compile against musl" with a quick
hack adding "#ifdef MUSL ... [omit feature or substitute in
fallback definition] ... #endif". In a best case, this would harm
functionality by leaving the feature out even in later musl versions
that support it; in reality, it's likely to actively break build (e.g.
with conflicting declarations/macros/etc.) if the feature is added to
musl.

Justification is idiotic because it doesn't consider cross-platform projects that need to work with many different CRT/libc libraries.