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().

win/Qt/qt_pre.h

@@ -17,14 +17,22 @@
 #undef min
 #undef max
 
+#if defined(__cplusplus)
+
+#ifdef __clang__
+#pragma clang diagnostic push
+#endif
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#endif
+/* the diagnostic pop is in qt_post.h */
+
+#ifdef __clang__
 /* disable warnings for shadowed names; some of the Qt prototypes use
    placeholder argument names which conflict with nethack variables
    ('g', 'u', a couple of others) */
-#ifdef __clang__
-#pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wshadow"
-#elif defined(__GNUC__) && defined(__cplusplus)
-#pragma GCC diagnostic push
+#elif defined(__GNUC__)
 #pragma GCC diagnostic ignored "-Wshadow"
 #endif
 
@@ -37,5 +45,26 @@
 #define QFM_WIDTH(foo) horizontalAdvance(foo)
 #endif
 
+#if __cplusplus >= 202002L
+/* c++20 or newer */
+#if QT_VERSION < 0x060000
+/*
+ * qt5/QtWidgets/qsizepolicy.h
+ * Qt5 header file issue under c++ 20
+ *
+ * warning: bitwise operation between different enumeration types 
+ * ‘QSizePolicy::Policy’ and ‘QSizePolicy::PolicyFlag’
+ * is deprecated [-Wdeprecated-enum-enum-conversion]
+ */
+#if defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wdeprecated-enum-enum-conversion"
+#endif
+#if defined(__clang__)
+#pragma clang diagnostic ignored "-Wdeprecated-enum-enum-conversion"
+#endif
+#endif  /* QT_VERSION < 0x060000 */
+#endif  /* __cplusplus >= 202002L */
+#endif /* __cplusplus */
+
 /*qt_pre.h*/