cmake: Add an option to enable/disable building of executables

Add the CMake option PNG_EXECUTABLES (on by default) in order to
allow or disallow the building of non-essential executable programs
associated with libpng.

Contributed-by: Alex Gaynor <alex.gaynor@gmail.com>
Contributed-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>
1 file changed