When building the embedded port on MinGW-w64, I receive the following
error:
fatal error: alloca.h: No such file or directory
MinGW-w64 (used on MSYS2) doesn't include `alloca.h`, but `alloca()` is
provided via `malloc.h` instead. And this fix is also needed for other
Windows build systems.
Signed-off-by: SiZiOUS <sizious@gmail.com>