Don't redefine asprintf when building for Android

[etcwilde]

Bionic includes a definition and implementation of asprintf. We should
not include our own on Android.

See 'include/stdio.h' and 'libc/stdio/stdio.cpp'

https://android.googlesource.com/platform/bionic/+/refs/heads/master/libc/include/stdio.h#288
https://android.googlesource.com/platform/bionic/+/refs/heads/master/libc/stdio/stdio.cpp#736

[etcwilde]

@swift-ci please test

[drodriguez]

/cc @buttaface FYI

[finagolfin]

LGTM, but maybe the similar #3021 should be merged instead?

[etcwilde]

That works for me too.

[etcwilde]

I should say that it works for me with caveats.
Digging through the macros. It's already defined if you have __USE_GNU enabled, or STDC_WANT_LIB_EXT2 set, so we shouldn't re-declare it in that case. @compnerd will likely have more comments.

[compnerd]

__STDC_WANT_LIB_EXT2__ is meant to be controlled by the user. However, __USE_GNU also triggers the extensions. Given that the GNU extensions enabled asprintf backdating before Swift, I think that @buttaface's suggestion is the right thing to do - we should simply remove the TARGET_OS_LINUX conditional. However, we should ensure that _GNU_SOURCE is defined (which I believe we do set) before we include stdio.h (or the appropriate header).