Remove various broken/useless features from the build system.

- Removed support for changing the build directory using configure.
  This can still be set using make BUILDPATH=foo.
- Removed support for SPLIT_CC builds. This is not documented
  anywhere and is quite useless as it doesn't work.
- Remove STARTSCRIPT from configure; always install the perl
  helper.
Clean up a bunch of legacy code in the Poll socketengine.