Some more #ifdef cleanups

src/cmd/ksh93/edit/edit.c,
src/cmd/ksh93/edit/history.c,
src/cmd/ksh93/sh/deparse.c:
- Remove experimental code protected by '#ifdef future'.
  No one is going to do anything with this, it's just clutter.

src/lib/libast/sfio/sfcvt.c:
- In 2021, it might be time to actually start using some C99
  features were available. Change two checks for a _c99_in_the_wild
  macro to actual checks for C99, enabling the use of fpclassify().

Resolves: https://github.com/ksh93/ksh/issues/219

src/lib/libast/sfio/sfcvt.c

@@ -160,7 +160,7 @@ int		format;		/* conversion format		*/
 			return SF_INF;
 		}
 #endif
-# if _c99_in_the_wild
+# if __STDC_VERSION__ >= 199901L
 #  if _lib_signbit
 		if (signbit(f))
 #  else
@@ -338,7 +338,7 @@ int		format;		/* conversion format		*/
 			return SF_INF;
 		}
 #endif
-#if _c99_in_the_wild
+#if __STDC_VERSION__ >= 199901L
 # if _lib_signbit
 		if (signbit(f))
 # else