Touch-ups for Regal build of mongoose

Fix for issue 8

Makefile for MinGW 4.7

Fix building of mongoose under FreeBSD.

According to flockfile(3) under Linux you need only this:

    #define _XOPEN_SOURCE

* http://man7.org/linux/man-pages/man3/flockfile.3.html

This also fixes the build under FreeBSD.

Allow debug tracing to be disabled in debug-mode build by defining DEBUG_TRACE at compile-time.

Removed the test for a directory name with only a backslash, because this causes problems on windows.

Removed the test for a directory name with only a backslash, because this causes problems on windows.

Added printf format checking macros. Changed mg_printf() to allocate the buffer on heap if it does not fit the local one.