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.

dialogs.cpp

@@ -803,7 +803,7 @@ EX namespace dialog {
     else
       addSlider(ne.sc.direct(ne.vmin), ne.sc.direct(*ne.editwhat), ne.sc.direct(ne.vmax), 500);
     addBreak(100);
-#if ISMOBILE==0
+#if !ISMOBILE
     addHelp(XLAT("You can scroll with arrow keys -- Ctrl to fine-tune"));
     addBreak(100);
 #endif