stb_vorbis.c - Detect `__NEWLIB__` for `alloca.h`

This is needed for `stb_vorbis.c` to compile for the Wii U using
devkitPro.

This should theoretically also fix compilation for the Nintendo
Switch, 3DS, and Wii (with devkitPro, that is) as they all also use
Newlib.

Newlib is also used by Cygwin:
https://cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=newlib/libc/include/alloca.h;h=5d36318914282280b353aed457e1b1f64947b584;hb=HEAD

And the Google Native Client:
https://chromium.googlesource.com/native_client/nacl-newlib/+/refs/heads/master/newlib/libc/include/alloca.h

As you can see from these links, these both provide `alloca.h` as
well, so it appears to be a safe guarantee that `alloca.h` is
available on Newlib.

stb_vorbis.c

@@ -577,7 +577,7 @@ enum STBVorbisError
    #if defined(_MSC_VER) || defined(__MINGW32__)
       #include <malloc.h>
    #endif
-   #if defined(__linux__) || defined(__linux) || defined(__EMSCRIPTEN__)
+   #if defined(__linux__) || defined(__linux) || defined(__EMSCRIPTEN__) || defined(__NEWLIB__)
       #include <alloca.h>
    #endif
 #else // STB_VORBIS_NO_CRT