Remove extra "within" from a function description.
Fix description of thud-env
Fix thud_abort and thud_abort_frame descriptions
Support portable installation

Add support for installations which can be moved around, e.g. for
installing into source distributions for use in tests:

* Add support for --disable-docs configure option, which disables
  installtion of documentation.
* Add support for --enable-lib-path configure option, which can be used
  to specify relative paths to installed library directory.