Fix for VPATH builds

ProTip: always `make distcheck` before release.

Bugs discovered by doing:

    ./bootstrap
    mkdir build && cd build
    ../configure
    make
    make DESTDIR="$PWD/inst" install

This commit makes fixes to:

  - check for and build xrectsel correctly
  - install zsh_completion correctly
Add substitution for PACKAGE_* output variables
Add "REPORTING BUGS" section to ffcast(1)