Make cabal flags manual

[tomsmeding]

Description
This patch changes all cabal flags to manual in accelerate.cabal.

Motivation and context
This prevents Cabal from automatically assigning flags in the constraint solver. This currently cannot result in issues: if any build configuration is valid, the default one is valid too. However, using 'manual' flags makes this future-proof against changes within Accelerate, and also changes within Cabal: with 'manual' we select the semantically correct type of flag.

This should not adversely affect anything, especially since the flags are only meant for Accelerate developers and testers anyway.

How has this been tested?
Accelerate still builds.

Types of changes
What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist
Go over all the following points, and put an x in all the boxes that apply. If you're unsure about any of these, don't hesitate to ask. We're here to help!

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed