Require C90 compliance

Assume all supported platforms implement C90, and therefore they
(correctly) implement atexit(), memmove(), setlocale(), strerror(),
and vprintf(), and have <float.h> and <limits.h>.

(Also remove the configure check testing that "do ... while (0)" works
correctly; the non-do/while-based version of G_STMT_START and
G_STMT_END was removed years ago, but the check remained. Also, remove
some checks that configure.ac claimed were needed for libcharset, but
aren't actually used.)

Note that removing the g_memmove() function is not an ABI break even
on systems where g_memmove() was previously not a macro, because it
was never marked GLIB_AVAILABLE_IN_ALL or listed in glib.symbols, so
it would have been glib-internal since 2004.

https://bugzilla.gnome.org/show_bug.cgi?id=710519

glib/gfileutils.c

@@ -2419,7 +2419,7 @@ g_path_get_dirname (const gchar *file_name)
 
   len = (guint) 1 + base - file_name;
   base = g_new (gchar, len + 1);
-  g_memmove (base, file_name, len);
+  memmove (base, file_name, len);
   base[len] = 0;
 
   return base;