Always use builtin alloca()

[sjmulder]

Fixes the build on NetBSD 9 on arm64.

GCC expands alloca() to __builtin_alloca() but only in nonstandard mode,
e.g. when -std=... is not supplied. In standards mode (with -std=...)
alloca() is left undefined by GCC. The C library may define it but it
also may not, as on NetBSD on arm64:

$ uname -srp
NetBSD 9.0 aarch64

$ cat alloca.c
#include <stdlib.h>
int main() { char *p = alloca(10); }

$ gcc alloca.c

$ gcc -std=c99 alloca.c
alloca.c:(.text+0xc): warning: Warning: reference to the libc
 supplied alloca(3); this most likely will not work. Please use the
 compiler provided version of alloca(3), by supplying the
 appropriate compiler flags (e.g. not -std=c89).
ld: alloca.c:(.text+0xc): undefined reference to `alloca'

The fix is to either not use standards mode (undesirable) or to
explicitly use the builtin, which is what this patch does.

This is also sufficient for Solarius/Illumos so that check and include
are removed.

[sjmulder]

Sorry for all the revisions – first pushed an older commit, then forgot to check Linux.

[leovilok]

We might want to check if this works in a non GNU environment, e.g. clang+musl.

[sjmulder]

Good point, successfully tested on macOS Catalina (10.15) with clang now.

[jarun]

Thank you!