Use runtime wcwidth() to check libc

We've been using #ifdefs and such to check for libc
Unicode 13 support at compile time. Instead, execute
a wcwidth() using a sextant. If it comes back -1, the
libc lacks support; disable sextants. Otherwise, let
it go. This ought allow us to support sextants on BSD
as soon as it has support; it furthermore allows us to
properly disable them on Linuxes lacking such support.
Closes #1289.

src/lib/terminfo.c

@@ -59,10 +59,10 @@ apply_term_heuristics(tinfo* ti, const char* termname){
   }else if(strstr(termname, "vte") || strstr(termname, "gnome")){
     ti->sextants = true; // VTE has long enjoyed good sextant support
   }
-// as of freebsd 12.1 / dragonfly 5.9, there's no libc support for sextants
-#if defined(__FreeBSD__) || defined(__DragonFly__)
-  ti->sextants = false;
-#endif
+  // run a wcwidth() to guarantee libc Unicode 13 support
+  if(wcwidth(L'🬸') < 0){
+    ti->sextants = false;
+  }
   return 0;
 }