Kipp Cannon authored Nov 11, 2019 and Patrick Godwin committed Dec 09, 2019

- this mostly reverts the rest of 70c88cd6
  "use private fields in pkgconfig files"
- the requirements of each of the gstlal libraries are not private, it is
  through these dependencies that the correct -I options are added when
  compiling things that #include the header files.  NOTE:  the header files
  in, for example, gstlal #include header files from other
  projects, header files that are not, in general, located in the default
  search paths of compilers.  pkg-config is required to determine the
  complete set of -I options, it does so using the "requires" field.
- 70c88cd6 "use private fields in pkgconfig
  files" didn't break gstlal builds because the gstlal packages,
  themselves, tend to have the same internal requirements as the exported
  requirements of other gstlal packages.  for example, using gstlal's
  header files requires you to add the -I options necessary to find all the
  gstreamer headers, which everything in gstlal-inspiral needs to add
  anyway, for its own reasons, so breaking gstlal's .pc file didn't break
  the gstlal-inspiral build.  it *does* break builds of other stuff,
  outside of these projects, that might try to link to gstlal but isn't a
  gstreamer based project itself.
- what this patch does do, is conduct an audit of what things are really
  #include'd in installed header files and only lists those things in the
  requires field instead of all libraries the project links against.