install: Teach CODE,SCRIPT modes to evaluate generator expressions
This also introduces CMP0087 which will keep the OLD behaviour of not evaluating generator expressions.
This change was first made by !2113 (merged) but was reverted by !2474 (merged) due to breaking existing projects. This MR re-introduces the change with a policy.
Fixes: #15785 (closed)
Topic-rename: install-code-script-genex