rtags: Fix configure phase errors

Add pkgconfig as buildinput, so that the install path is correctly set
with cmake. PkgConfig is an optional dependency for rtags, but they
say it's necessary if you want to replace the prefix with
CMAKE_INSTALL_PREFIX. See:
https://github.com/Andersbakken/rtags/blob/caad9ac494f09960bd60a00061279efacc8d1d26/cmake/BashCompletion.cmake#L13

Furthermore, I let the configurePhase of the rtags emacs package be a
noop.