xerces-c: add variant for transcoder, rework config flags. (#9393)

* xerces-c: Add variant for choice of transcoder (gnuiconv, iconv, icu,
  macos, windows).  It's important to specify a --enable-transcoder
  option on the configure line or else xerces may make a different
  choice when multiple transcoders are available.
* Pass the compile flags to configure.  For cflags and cxxflags, this is
  necessary to respect the value from the spack install line.
  Otherwise, xerces (and any autotools package) will choose a default
  value that overrides the spack compiler wrapper.
* Add xerces version 3.2.2.
* icu4c needs the --enable-rpath configure option on Darwin