Windows 10 Pro version 1703 with AMD Radeon HD 7970 driver 17.9.1
Broken: 2.79.1 6f633de as well as the stable release version 2.79 5bd8ac9
Worked: 2.78c stable release
Short description of error
Blender 2.79 won't let me select my AMD GPU for rendering with Cycles.
Exact steps for others to reproduce the error
I simply downloaded the new stable version today and installed it over 2.78c and copied the settings from the previous version on the splash screen. Noticed that GPU Compute was greyed out so I checked the User Preferences > System > Cycles Compute Device setting and it was simply "None". Tried restoring factory settings, reinstalling Blender, deleting all Blender settings from AppData, installing latest test build and upgrading my AMD drivers but it's all the same.
I downloaded the 2.78c stable version and GPU rendering works fine like always.