Add giftware license from Allegro 4.

This seems correct since a lot of the code in this project comes
straight from Allegro 4.
scons: Search for pkg-config files with "-5" suffix instead of "-5.1".
scons: Link with allegro_acodec.
allegro4-to-5: Enable compilation with Allegro 5.0.

Some things won't work correctly, though.
allegro4-to-5: Call constructor functions manually if necessary.

Since the header file cleanup in Allegro 5.1.7, ALLEGRO_USE_CONSTRUCTOR
will not be defined as a side-effect of including allegro5.h.
allegro4-on-5: Suppress warnings about uninitialised fields.
allegro4-to-5: Implement set_mouse_sprite(NULL).
allegro4-to-5: Improve convert_8bit.
allegro4-to-5: Fix bitmap conversion of color fonts.
allegro4-to-5: Implement -1 color to text drawing routines.
allegro4-to-5: Flip screen after textout_centre*, textout_right*.
allegro4-to-5: Fix formatting.
allegro4-to-5: Fix angle in pivot_sprite.
allegro4-to-5: Implement rotate_sprite, rotate_sprite_v_flip.
allegro4-to-5: Implement stretch_sprite.
allegro4-to-5: Add 'convert_8bit' as an API extension.

Also use _rgb_scale_6 to scale up 6-bit colours.
allegro4-to-5: Add CONVERT_8BIT_INTENSITY.
allegro4-to-5: Add proper mutex/condvar around keybuffer.
allegro4-to-5: Do NOT set target bitmap in bg thread for keyboard_lowlevel_callback.

The main thread is likely already targeting a bitmap of the display, if
not exactly the same bitmap.  Besides which, it is not allowed use the
same rendering context (tied to the display) from multiple threads at
once.
allegro4-to-5: Maintain key_shifts for flags other than SHIFT.