automatic BuildRequires for source RPM: for every FILEPATH and
"Have library XXX" cached variable, detect what rpm package it comes from
and add it to the list of dependencies.

That is, the source RPM will BuildRequire all those packages that
were found by cmake when the source RPM was built. Presumably, our
CMakeLists.txt won't check for libraries that aren't needed for a build.

It supports libraries/executables/files found with
  FIND_LIBRARY
  FIND_FILE
  FIND_PROGRAM
  CHECK_LIBRARY_EXISTS