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

gio/gbufferedinputstream.c

@@ -639,7 +639,7 @@ compact_buffer (GBufferedInputStream *stream)
 
   current_size = priv->end - priv->pos;
 
-  g_memmove (priv->buffer, priv->buffer + priv->pos, current_size);
+  memmove (priv->buffer, priv->buffer + priv->pos, current_size);
 
   priv->pos = 0;
   priv->end = current_size;

gio/gbufferedoutputstream.c

@@ -431,7 +431,7 @@ flush_buffer (GBufferedOutputStream  *stream,
   count = priv->pos - bytes_written;
 
   if (count > 0)
-    g_memmove (priv->buffer, priv->buffer + bytes_written, count);
+    memmove (priv->buffer, priv->buffer + bytes_written, count);
   
   priv->pos -= bytes_written;