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.

control.cpp

@@ -1021,7 +1021,7 @@ EX void mainloop() {
 #endif
   }
 
-#if ISMOBILE==1
+#if ISMOBILE
 EX void displayabutton(int px, int py, string s, int col) {
   // TMP
   int siz = vid.yres > vid.xres ? vid.fsize*2 : vid.fsize * 3/2;