Raise the minimum required CMake version to 3.5.
Add the configuration option `PNGMINUS_USE_SYSTEM_PNG` for compiling
and linking with the system libpng library instead of the internal one.
Remove the old configuration option `PNGMINUS_USE_STATIC_LIBRARIES`.
When using the internal libpng (via `PNGMINUS_USE_SYSTEM_PNG=OFF`),
simply enforce static linking and produce single-file executables.
Rename the scripts "png2pnm.sh" (etc.) to "test_png2pnm.sh" (etc.),
to make it obvious that they are test drivers, not program launchers.
Add a .gitignore file for project-specific build and test artifacts.
