The Allegro library provides C/C++ programmers low level routines commonly
needed in game programming, such as input, graphics, midi, sound effects, and
timing. It is cross platform and works with many different compilers.
Originally developed by Shawn Hargreaves, it is now a group project with
contributions from all over the world! Some of its key features include:
    * Ease of Use - comes with comprehensive documentation and examples
    * Extensibility - if the built-in functionality isn't enough, there
      are many add-ons available
    * Cross Platform - without changing a single line of code, you can
      create versions for Windows, OS X, Linux, DOS, and others! Works
      with many compilers, including DJGPP and VC++
    * Open Source - anyone can contribute, including you!
    * Free - it won't cost you a dime, and there are no restrictions on
      its usage

You can disable native_dialog addon by passing this option :
ALLEGRO_NATIVE_DIALOG=off ./allegro.Slackbuild

Since it depends on gtk3, and slackware-14.2's gtk3 is too old, this addon
prevents allegro 5.2.7+ to build on slackware-14.2.
So if you want, or need, to build allegro 5.2.7.0 on slackware-14.2 you have
to disable this addon by running this command :
VERSION=2.5.7.0 ALLEGRO_NATIVE_DIALOG=off ./allegro.Slackbuild

On slackware-current, the simple version bump is enough, and the native_dialog
addon works pretty fine :
VERSION=2.5.7.0 ./allegro.Slackbuild