Fix bugs

[nmcclatchey]

Bug fixes:

• Restore StandardLayout property to timed_mutex and recursive_timed_mutex, which had been lost in a previous commit. Add regression tests for this.
• Apply an alternate approach to resolving multiple-include errors; this fixes problems with compiling "mingw.future.h" that were not detected by tests.
• Ensure that the StandardLayout property is satisfied by condition_variable and condition_variable_any. Add regression tests for this.
• Cause the Windows XP condition_variable and condition_variable_any to throw an exception when construction fails, rather than silently failing.
• Include <functional> in "mingw.future.h" to ensure that std::hash is defined.
• Include all referenced headers in "mingw.thread.h" to ensure that changes to MinGW's implementation do not break the library.

Non-bug improvements:

• Statically initialize condition_variables for Vista and higher. This may slightly improve performance when they are created.
• Reduce include burdens in individual files by omitting headers that won't be used under the given options.
• Remove <algorithm> dependency, to improve compile times.