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.

system.cpp

@@ -1068,7 +1068,7 @@ EX void saveStats(bool emergency IS(false)) {
   
   fprintf(f, "\n\n\n");
   
-#if ISMOBILE==0
+#if !ISMOBILE
   DEBB(DF_INIT, ("Game statistics saved to ", scorefile));
   addMessage(XLAT("Game statistics saved to %1", scorefile));
 #endif
@@ -1556,7 +1556,7 @@ EX void finishAll() {
   saveStats();
 #endif
   clearMemory();
-#if ISMOBILE==0
+#if !ISMOBILE
   cleargraph();
 #endif