The introduced checks were not correct in two respects:
 * It was checked whether the source encoding of the string matches
   the internal encoding, while the actually relevant encoding is
   the *target* encoding.
 * Even if the correct encoding is used, the checks are still too
   conservative. Just because something is not a "Unicode-encoding"
   does not mean that it does not map any non-ASCII characters.

I've reverted the added checks and instead adjusted mbfl_convert
to first try to use the provided substitution character and if
that fails, perform the fallback to '?' at that point. This means
that any codepoint mapped in the target encoding should now be
correctly supported and anything else should fall back to '?'.

The HTML decoding filter uses the `opaque` member of mbfl_convert_filter
as buffer, but there was no copy constructor defined, what caused double
frees when the filter is copied (what happens multiple times in mb_strcut(),
for instance).

configure.ac was introduced in 2001 with automake-1.15 and autoconf-2.50
to replace the file named configure.in.
Autotools is preparing to remove configure.in in Automake 2.0.
All new software should be using configure.ac.
This also fixes Bug #69770 where extensions are creating configure.in

Signed-off-by: Brian Evans <grknight@gentoo.org>

* PHP-5.6:
  fix C89 compat

* PHP-5.6:
  Add more mbfl string size checks (bug #73505)