Simplify libxmlrpc bundling and building

Simplify libxmlrpc bundling and building

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.