scripts: Add general-purpose utilities for continuous integration

Currently, libpng supports three different types of build automation:
the GNU autotools, CMake, and a legacy of hand-made makefiles.

In order to simplify the continous integration of all of the above
build options, we introduce the following scripts:
 * CI utility for the Autotools build
 * CI utility for the CMake build
 * CI utility for the legacy makefiles
4 files changed