_alloca is not part of msvcrt (and therefore not found if used), and tcc has
an internal implementation for alloca for x86[_64] since d778bde7 - initally
as _alloca and later changed to alloca. Use it instead.
which live in msvcrt.dll and need __declspec(import) which
works by now.
Also:
- _mingw.h: conditionally define WIN32_LEAN_AND_MEAN
- malloc.h: don't undef alloca