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.

src/wizard.c

@@ -341,6 +341,8 @@ choose_stairs(xchar *sx, xchar *sy)
     }
 }
 
+DISABLE_WARNING_UNREACHABLE_CODE
+
 int
 tactics(struct monst *mtmp)
 {
@@ -432,6 +434,8 @@ tactics(struct monst *mtmp)
     return 0;
 }
 
+RESTORE_WARNINGS
+
 /* are there any monsters mon could aggravate? */
 boolean
 has_aggravatables(struct monst *mon)