[Bug 54214] Document a way to pass optimization levels to mingw through CROSSCFLAGS in configure --help

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=54214

Shmerl  changed:

What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|No way to pass optimization |Document a way to pass
                   |levels in x86_64_CFLAGS to  |optimization levels to
                   |mingw                       |mingw through CROSSCFLAGS
                   |                            |in configure --help