Move all defaulting-of-CAP_FOO to sysconfig.h.

- The phrase `#ifdef CAP_` should never appear anywhere.

- The phrase `#ifndef CAP_` should appear only in sysconfig.h.

- The phrases `#if CAP_` and `#if !CAP_` may appear wherever,
    as long as "sysconfig.h" is included first.

The rules for `CAP_FOO` equally apply to `ISFOO`.

There are many one-off macros still tested with `#ifdef`,
including `HAVE_ACHIEVEMENTS`, `PRINT_ACHIEVEMENTS`,
`FAKEWEB`, `FAKE_SDL`, `EASY`, and `WHATEVER`. I don't
have much grasp on what these are used for or how they're
configured, so I'm leaving them alone.

items.cpp

@@ -467,7 +467,7 @@ EX void gainItem(eItem it) {
   if(chaosmode && gold() >= 300)
     achievement_gain_once("CHAOS", rg::chaos);
 
-#if ISMOBILE==1
+#if ISMOBILE
   if(g < lastsafety + R30*3/2 && g2 >= lastsafety + R30*3/2)
     addMessage(XLAT("The Orb of Safety from the Land of Eternal Motion might save you."));
 #endif