- use `Requires.private` in pkgconfig files to stop forcing downstream
  packages to link against everything upstream
- added `pkgconfig` as `BuildRequires` for RPMs
- bumped required version of pkg-config to 0.18.0

gstlal-calibration-1.2.3 release.  Includes a minor bug fix so that all needed plugins get built and packaged.

gstlal-calibration:  Added debugging element splitcounter for printing buffer timestamps and offsets to screen and file.

- new package cmath: elements that perform unary operations and handle complex and real floating point arithmetic. Elements included are cabs, cexp, cln, clog, clog10, and cpow.
- new element lal_trackfrequency: base transform element that attempts to measure the loudest frequency in a signal.

gstlal-calibration:  adjust gstreamer plugin .so filenames on gstreamer 1.14+ to work around changes to name mangling used by the plugin scanner