Add color calibration support via colord
configure.ac: Require autoconf 2.69

From fedora bug report:
Support for the ARM 64 bit CPU architecture (aarch64) was introduced in
autoconf 2.69.  redshift appears to use an earlier version of
autoconf, preventing its being built.
(Thanks Miloš Komarčević, Dennis Gilmore)
Fix slight wobble in temperature caused by float.

The transition variable 'adjustment_alpha' is a float. Redshift will wobble between 6499 and 6500K as the float runs out of precision. Using a double seems to solve the issue.
(Thanks to Sean Hildebrand)
colorramp: Fix slight flicker caused by improper scaling of gamma ramp values

Values generated by colorramp_fill() will be a maximum of i * 256 - 1, due  to the use of UINT16_MAX. Simplest fix ought to be using (UINT16_MAX+1), which is safe, as the maximum value (255 * 256) still fits in a uint16.
(Thanks to Sean Hildebrand)
location-geoclue: Fix crash when starting geoclue provider. (Thanks to Maks Verver)
Fix typos (Benjamin Kerensa)
Bump version to 1.8
Add .gitignore
Update HACKING document
colorramp: Use new color ramp table provided by Ingo Thies.

This table is calculated by the actual Planckian locus below 5000K, and by CIE daylight standard above 5000K.
Also, the new table applies proper gamma correction according to sRGB.
More information can be found in README-colorramp.