RFC: https://wiki.php.net/rfc/tostring_exceptions

And convert some object to string conversion related recoverable
fatal errors into Error exceptions.

Improve exception safety of internal code performing string
conversions.

Closes GH-4221.

Normalization include:
- Use dnl for everything that can be ommitted when configure is built in
  favor of the shell comment character # which is visible in the output.
- Line length normalized to 80 columns
- Dots for most of the one line sentences
- Macro definitions include similar pattern header comments now

The xmlrpc library is a forked, modified and bundled into the PHP source
code. Instead of having a separate *nix build system files in the xmlrpc
library this patch removes unneeded build checks.

- AC_PROG_CC is not needed to be called by the extension itself since it
  is already part of the main configure.ac and the phpize's
  configure.ac.
- UNDEF_THREADS_HACK is not used in the current codebase.
- Symbols defined by XMLRPC_HEADER_CHECKS are not used in the current
  code.
- Symbols defined by XMLRPC_FUNCTION_CHECKS are not used in the current
  code.
- AC_PROG_RANLIB is already done by the TSRM's files and isn't needed
  for the xmlrpc extension to work.
- AC_PROG_LN_S is already done by the PHP main configure.ac and also
  isn't needed by the xmlrpc extension to be built.