g++-12 bits, mostly Qt5 related

I forced a test compile to -std=c++20 mostly to see what we would
be up against. There was only a small number of things and they
are corrected in this commit.

c++20 has some issues with comparisons and bit twiddling between
different enums.

The vendor-supplied Qt5 header files triggered some of those issues as
well, so the qt_pre.h and qt_post.h NetHack header files were adjusted
to make those new warnings go away.  I have not tested Qt6 under the
new compiler and c++ version yet.

Because there are multiple pragmas in qt_pre.h now, the conditional
ifdef structure in there was modified a little to make maintenance
simpler and have a single pragma push at the top. The pragma pop
comes after the Qt vendor-supplied header files, and is done
in qt_post.h.

The display.h macro cmap_to_glyph() was used in
a Qt c++ file and triggered a series of warnings because of that.
Rather than write c++20-friendly versions of those macros, the
simple fix is to provide a function on the C side of things
to front the cmap_to_glyph() macro, so fn_cmap_to_glyph()
was added.

Also thrown into this commit, PatR picked up on the fact that for
yesterday's new warning in qt_menu.cpp, the compiler had correctly
picked up on the fact that the format range of the variable 'cash'
had been correctly upper-capped at 999999999L in the warning message
because of an assignment prior. He suggested that perhaps by also adding
    if (cash < 0)
       cash = 0;
the warning might be eliminated altogether.
After a test, that was proven to be correct, so yesterday's
more-kludgy change is reverted and replaced with that variable
variable restriction ahead of the snprintf().

include/extern.h

@@ -443,6 +443,7 @@ extern void unset_seenv(struct rm *, int, int, int, int);
 extern int warning_of(struct monst *);
 extern void map_glyphinfo(xchar, xchar, int, unsigned, glyph_info *);
 extern void reset_glyphmap(enum glyphmap_change_triggers trigger);
+extern int fn_cmap_to_glyph(int);
 
 /* ### do.c ### */