Core: Avoid short read failures on broken descriptors

* See http://marc.info/?t=135132844600001&r=1&w=2
Autotools: Carry over 2012.10.23 libusb changes

* Use LIBS instead of PC_LIBS_PRIVATE
* Move THREAD_CFLAGS & VISIBILITY_CFLAGS into AM_CFLAGS
* Use AC_SEARCH_LIBS(clock_gettime) for pthreads on Linux
Tests: Add libusbx stress test

See https://github.com/tobygray/libusbx/tree/testing as well
as http://libusbx.1081486.n5.nabble.com/Libusbx-devel-Crashes-tt433.html#a438
Core: Fix incorrect reference counting of default context when first created.

* See http://libusbx.1081486.n5.nabble.com/Libusbx-devel-stress-program-crashes-in-libusb-exit-NULL-tp474p477.html
Windows: Add VS2012 solution files

* As well as try to clean up MS's MSVC mess...
Core: Filter out 8-bit characters in libusb_get_string_descriptor_ascii

* Closes #68
WinCE: Add backend and build files for Windows CE backend.

This backend requires the CEUSBKWrapper driver to be installed
on the WinCE device. Without this no devices will be listed.

The CEUSBKWrapper driver is licensed under the LGPL 2.1 and can be
found at https://github.com/RealVNC/CEUSBKWrapper

Co-authored-by: Simon Haggett <simon.haggett@realvnc.com>
WinCE: Adding files used by Windows CE build to .gitignore.
WinCE: Adding support for WinCE backend to common files and headers.
WinCE: Updating time function calls to use appropriate APIs on WinCE.
WinCE: Adding registry based environment variables.

To set environment variables for libusb they should be values
contained in HKLM\Software\libusb\environment

For example the following in a .reg file would enable full debug logging:

[HKEY_LOCAL_MACHINE\SOFTWARE\libusb\environment]
"LIBUSB_DEBUG"="4"
WinCE: Correcting error message in config.h
WinCE: Adding newlines at end of files.
WinCE: Removing unnecessary #ifdef for timerfd.
WinCE: Removing dummy header files in favour of wrapping #includes with #ifndef WINCE
WinCE: Sharing common code between control and bulk transfers.
WinCE: Moving getenv functionality into missing.c and missing.h.
WinCE: Renaming and restructuring build directory.
WinCE: Adding building of xusb example to WinCE builds.

Also fixed include paths for listdevs example.
WinCE: Reorganising layout of files in VS2005 Solution Explorer.