Remove wxUSE_COMPILER_TLS and always use C++11 thread_local

Don't use compiler-specific __thread and use the standard keyword
instead.

Keep the existing TLS-related macros for compatibility, but remove our
custom implementation of them, so that they're always trivial now.

docs/doxygen/mainpages/const_wxusedef.h

@@ -84,7 +84,6 @@ library:
 @itemdef{wxUSE_COLOURPICKERCTRL, Use wxColourPickerCtrl class.}
 @itemdef{wxUSE_COMBOBOX, Use wxComboBox class.}
 @itemdef{wxUSE_COMBOCTRL, Use wxComboCtrl class.}
-@itemdef{wxUSE_COMPILER_TLS, Can be set to 0 to prevent using compile thread-specific variables support.}
 @itemdef{wxUSE_CONFIG, Use wxConfig and related classes.}
 @itemdef{wxUSE_CONFIG_NATIVE, When enabled use native OS configuration instead of the wxFileConfig class.}
 @itemdef{wxUSE_CONSOLE_EVENTLOOP, Enable event loop in console programs.}