This change addresses two problems:

  1. The Autotools build system was shipping liblab_gamut.so and liblab_gamut.pc
     but not lab_gamut.h. As a result, end users had no header defining the API
     of liblab_gamut and could not easily use it.

  2. The CMake build system was shipping liblab_gamut.pc but defined no
     liblab_gamut. This resulted in a pkg-config file that referred to a
     non-existent library.

There are no known external users of liblab_gamut. So this change slims the
Graphviz distribution by removing it from the installed artifacts.

Gitlab: closes #2489