[libmicrohttpd] Remove plibc?

[Evgeny Grin]

Hi!

The main advantage of plibc for MHD is ability to freely mix pipe and socket 
file descriptors on Win32, especially for select().
As I see MHD don't use anymore pipes on win32 systems.
If only socket FDs is used, then plibc can be finally removed.
Just remeber to use specialized function for socket FDs (for example 
closesocket() instead of close()). That can be easly implemented by #define 
MHD_closesocket.
This allow to simpler build x64 port on win32.

Best Wishes,
Evgeny