libclc: breaking changes in release/21.x branch

[mgorny]

I've found two serious breakages introduced in libclc over the last two weeks:

• 8a7a648 which broke standalone builds by moving set() into a subdirectory
• 81e6552 which made libclc install absolute symlinks to the build directory

For the first one I've submitted a fix in #149657. For the second one, I don't have a fix.

In either case, I think these last minute changes should be reverted from 21.x branch.

[mgorny]

Actually, to restore libclc to a working state, I need to revert the five most recent CMake-related changes:

• 8a7a648 to fix building
• 9b5959d as a prerequisite for the subsequent reverts
• 222e795 as a prerequisite for the subsequent reverts
• 81e6552 to fix installing broken symlinks
• 85d09de to fix symlinks in the build directory that break tests