suppress a particular warning for an individual function; useful for non-gcc

Microsoft and other non-GNU compilers don't recognize gcc tricks
like  /*NOTREACHED*/ to suppress individual warnings. clang recognizes most
of them because it tries to be gcc-compatible. Because of that, a lot of
potentially useful warnings have had to be completely suppressed in the
past in all source files when using the non-gcc compatible compilers.

Now that the code is C99, take advantage of a way to suppress warnings for
individual functions, a big step up from suppressing the warnings
altogether.

Unfortunately, it does require a bit of ugliness caused by the
insertion of some macros in a few spots, but I'm not aware of
a cleaner alternative that still allows warnings to be enabled
in general, while suppressing a warning for known white-listed
instances.

Prior to the warning-tiggering function, place whichever one of
the following is needed to suppress the warning being encountered:

DISABLE_WARNING_UNREACHABLE_CODE
DISABLE_WARNING_CONDEXPR_IS_CONSTANT

After the warning-triggering function, place this:

RESTORE_WARNINGS

Under the hood, the compiler-appropriate warning-disabling
mechanics involve the use of C99 _Pragma, which can be used
in macros.

For unrecognized or inappropriate compilers, or if
DISABLE_WARNING_PRAGMAS is defined, the macros expand
to nothing.

include/global.h

@@ -165,6 +165,8 @@ extern struct cross_target_s cross_target;
 #include "ntconf.h"
 #endif
 
+#include "warnings.h"
+
 /* amiconf.h needs to be the last nested #include of config.h because
    'make depend' will turn it into a comment, hiding anything after it */
 #ifdef AMIGA

include/warnings.h

@@ -0,0 +1,74 @@
+/* NetHack 3.7	warnings.h	$NHDT-Date: 1596498562 2020/08/03 23:49:22 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.24 $ */
+/* Copyright (c) Michael Allison, 2021. */
+
+#ifndef WARNINGS_H
+#define WARNINGS_H
+
+/*
+ * If ENABLE_WARNING_PRAGMAS is defined, the checks for various
+ * compilers is activated.
+ *
+ * If a suitable compiler is found, STDC_Pragma_AVAILABLE will be defined.
+ * When STDC_Pragma_AVAILABLE is not defined, these are defined as no-ops:
+ *     DISABLE_WARNING_UNREACHABLE_CODE
+ *     DISABLE_WARNING_CONDEXPR_IS_CONSTANT
+ *       ...
+ *     RESTORE_WARNINGS
+ *
+ */
+
+#if !defined(DISABLE_WARNING_PRAGMAS)
+#if defined(__STDC_VERSION__)
+#if __STDC_VERSION__ >= 199901L
+#define ACTIVATE_WARNING_PRAGMAS
+#endif /* __STDC_VERSION >= 199901L */
+#endif /* __STDC_VERSION */
+#if defined(_MSC_VER)
+#ifndef ACTIVATE_WARNING_PRAGMAS
+#define ACTIVATE_WARNING_PRAGMAS
+#endif
+#endif
+
+#ifdef ACTIVATE_WARNING_PRAGMAS
+#if defined(__clang__)
+#define DISABLE_WARNING_UNREACHABLE_CODE \
+                           _Pragma("clang diagnostic push") \
+                           _Pragma("clang diagnostic ignored \"-Wunreachable-code\"")
+#define DISABLE_WARNING_CONDEXPR_IS_CONSTANT
+#define RESTORE_WARNINGS _Pragma("clang diagnostic pop")
+#define STDC_Pragma_AVAILABLE
+
+#elif defined(__GNUC__)
+/* unlike in clang, -Wunreachable-code does not function in later versions of gcc */
+#define DISABLE_WARNING_UNREACHABLE_CODE \
+                           _Pragma("GCC diagnostic push") \
+                           _Pragma("GCC diagnostic ignored \"-Wunreachable-code\"")
+#define DISABLE_WARNING_CONDEXPR_IS_CONSTANT
+#define RESTORE_WARNINGS _Pragma("GCC diagnostic pop")
+#define STDC_Pragma_AVAILABLE
+
+#elif defined(_MSC_VER)
+#define DISABLE_WARNING_UNREACHABLE_CODE \
+                           _Pragma("warning( push )") \
+                           _Pragma("warning( disable : 4702 )")
+#define DISABLE_WARNING_CONDEXPR_IS_CONSTANT \
+                           _Pragma("warning( push )") \
+                           _Pragma("warning( disable : 4127 )")
+#define RESTORE_WARNINGS _Pragma("warning( pop )")
+#define STDC_Pragma_AVAILABLE
+
+#endif /* various compiler detections */
+#endif /* ACTIVATE_WARNING_PRAGMAS */
+#else  /* DISABLE_WARNING_PRAGMAS */
+#if defined(STDC_Pragma_AVAILABLE)
+#undef STDC_Pragma_AVAILABLE
+#endif
+#endif /* DISABLE_WARNING_PRAGMAS */
+
+#if !defined(STDC_Pragma_AVAILABLE)
+#define DISABLE_WARNING_UNREACHABLE_CODE
+#define DISABLE_WARNING_CONDEXPR_IS_CONSTANT
+#deifne RESTORE_WARNINGS
+#endif
+
+#endif /* WARNINGS_H */