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.

rogueviz/rogueviz.cpp

@@ -913,10 +913,6 @@ void close() {
   relmatrices.clear();
   }
 
-#ifndef CAP_RVSLIDES
-#define CAP_RVSLIDES (CAP_TOUR && !ISWEB)
-#endif
-
 #if CAP_COMMANDLINE
 int readArgs() {
   using namespace arg;