Support use as a Windows program outside of Cygwin

If `getenv("SHELL")` is `NULL`, use the function `win32_setenv_shell()`
added to `osdep-win32-cpp.cpp` to set it using the command line of the
parent process, which is retrieved using WMI WIN32 API.

This is a C++ file because the OLE/WMI API is only available for C++.
Adjust the autotools code to add this file and link the necessary
Windows DLLs.

Include some fixed/missing MINGW headers necessary to compile this file.
This will be fixed in the relevant places and they will be removed.

Add a new macro WIN32_PLATFORM for Windows specific functionality, currently
Cygwin and MSYS2, in the future for the native port as well.

When spawning commands using the shell, check for cmd.exe on Windows and
use the `/c` switch, otherwise use `-c` which works for PowerShell,
MSYS2, Cygwin and Git Bash etc..

Adjust code that uses `/tmp/` to use
`$env:USERPROFILE/AppData/Local/Temp/` outside of a Cygwin virtual
filesystem when `/tmp/` is not available, add the function
`win32_get_tmpdir()` and related functions to `osdep-win32.c` for this.

Use `getenv("USERPROFILE")` when `getenv("HOME")` is `NULL`.

When outside of a Cygwin virtual filesystem, use
`C:\ProgramData\tmux\tmux.conf:$USERPROFILE\.tmux.conf:$LOCALAPPDATA\tmux\tmux.conf`
as the config search order.

Use the ncurses term-driver with `TERM="#win32con"` when a terminfo
database is not available. This will require patches to ncurses as well
as MSYS2 and Cygwin to work.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

third_party/mingw_headers/new.h

@@ -0,0 +1,45 @@
+/**
+ * This file has no copyright assigned and is placed in the Public Domain.
+ * This file is part of the mingw-w64 runtime package.
+ * No warranty is given; refer to the file DISCLAIMER.PD within this package.
+ */
+#ifndef _INC_NEW
+#define _INC_NEW
+
+#ifdef __cplusplus
+
+#include <new>
+
+#include <crtdefs.h>
+
+#pragma push_macro("new")
+#undef new
+
+#ifndef __NOTHROW_T_DEFINED
+#define __NOTHROW_T_DEFINED
+#endif
+
+#ifndef __PLACEMENT_NEW_INLINE
+#define __PLACEMENT_NEW_INLINE
+#endif
+
+_CRTIMP int __cdecl _query_new_mode(void);
+_CRTIMP int __cdecl _set_new_mode(int _NewMode);
+
+#ifndef _PNH_DEFINED
+typedef int (__cdecl *_PNH)(size_t);
+#define _PNH_DEFINED
+#endif
+
+_CRTIMP _PNH __cdecl _query_new_handler(void);
+_CRTIMP _PNH __cdecl _set_new_handler(_PNH _NewHandler);
+
+#ifndef _NO_ANSI_NH_DEFINED
+#define _NO_ANSI_NEW_HANDLER ((new_handler)-1)
+#define _NO_ANSI_NEW_HANDLER_M ((_new_handler_m)-1)
+#define _NO_ANSI_NH_DEFINED
+#endif
+
+#pragma pop_macro("new")
+#endif
+#endif