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/do_name.c

@@ -335,6 +335,8 @@ gloc_filter_done(void)
     }
 }
 
+DISABLE_WARNING_UNREACHABLE_CODE
+
 static boolean
 gather_locs_interesting(int x, int y, int gloc)
 {
@@ -402,6 +404,8 @@ gather_locs_interesting(int x, int y, int gloc)
     return FALSE;
 }
 
+RESTORE_WARNINGS
+
 /* gather locations for monsters or objects shown on the map */
 static void
 gather_locs(coord **arr_p, int *cnt_p, int gloc)