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.

glhr.cpp

@@ -24,10 +24,6 @@ EX void glError(const char* GLcall, const char* file, const int line) {
     }
   }
 
-#ifndef CAP_VERTEXBUFFER
-#define CAP_VERTEXBUFFER (ISWEB)
-#endif
-
 #if HDR
 #if CAP_SHADER && CAP_NOSHADER
 #define WITHSHADER(x, y) if(glhr::noshaders) y else x