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.

quit.cpp

@@ -122,7 +122,7 @@ EX hint hints[] = {
     []() { return true; },
     []() { 
       dialog::addInfo(XLAT(
-#if ISMOBILE==1
+#if ISMOBILE
         "The 'world overview' shows all the lands in HyperRogue."
 #else
         "Press 'o' to see all the lands in HyperRogue."
@@ -457,12 +457,12 @@ EX void showMission() {
       dialog::addItem(XLAT("inventory"), 'i');
     if(racing::on)
       dialog::addItem(XLAT("racing menu"), 'o');
-    #if ISMOBILE==0
+#if !ISMOBILE
     dialog::addItem(XLAT(quitsaves() ? "save" : "quit"), SDLK_F10);
-    #endif
-    #if CAP_ANDROIDSHARE
+#endif
+#if CAP_ANDROIDSHARE
     dialog::addItem(XLAT("SHARE"), 's'-96);
-    #endif
+#endif
     }
   dialog::addItem(XLAT("message log"), 'l');