In P0941, the committee standardized a bunch of feature test macros.
[ I think it's a bad idea, but whatever. ]
This patch puts the existing libc++ feature test macros in <version>, where P0941 specifies, and makes sure that all the correct headers include <version>. It also introduces a full set of tests, one per header file that might get something defined.
At the moment, most of those tests are empty, since libc++ does not define many feature test macros.
But now it's easy to add more. Put them in <version> and update the appropriate tests. Helpfully, the list is at the top of each of the test files.