Define yy_current_buffer for backward compatibility.
Allow YY_NO_UNPUT to disable unput() for backward compatibility.
Enable GNU m4 compatibility mode.
Connect flex 2.5.37 to the build and bump __FreeBSD_version.
Avoid signed overflow in error handling code.

Reviewed by:	cperciva, bjk
Add <uchar.h>.

The <uchar.h> header, part of C11, adds a small number of utility
functions for 16/32-bit "universal" characters, which may or may not be
UTF-16/32. As our wchar_t is already ISO 10646, simply add light-weight
wrappers around wcrtomb() and mbrtowc().

While there, also add (non-yet-standard) _l functions, similar to the
ones we already have for the other locale-dependent functions.

Reviewed by:	theraven