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.

menus.cpp

@@ -230,7 +230,7 @@ EX void showMainMenu() {
   dialog::addItem(XLAT(inSpecialMode() ? "reset special modes" : "back to the start menu"), 'R');
   
   string q;
-  #if ISMOBILE==1
+  #if ISMOBILE
   dialog::addItem(XLAT("visit the website"), 'q');
   #else
   q = quitsaves() ? "save" : "quit"; 
@@ -248,7 +248,7 @@ EX void showMainMenu() {
   if(inv::on)
     dialog::addItem(XLAT("inventory"), 'i');    
 
-#if ISMOBILE==1
+#if ISMOBILE
 #if CAP_ACHIEVE
   dialog::addItem(XLAT("leaderboards/achievements"), '3'); 
 #endif
@@ -305,7 +305,7 @@ EX void showMainMenu() {
 #endif
     else if(sym == SDLK_ESCAPE) 
       showMissionScreen();
-  #if ISMOBILE==1
+  #if ISMOBILE
   #ifdef HAVE_ACHIEVEMENTS
     else if(NUMBERKEY == '3') {
       achievement_final(false);
@@ -382,7 +382,7 @@ EX void enable_cheat() {
   else if(!cheater) dialog::cheat_if_confirmed([] {
     cheater++;
     addMessage(XLAT("You activate your demonic powers!"));
-#if ISMOBILE==0
+#if !ISMOBILE
     addMessage(XLAT("Shift+F, Shift+O, Shift+T, Shift+L, Shift+U, etc."));
 #endif
     popScreen();