cmake: Modulize our TLS & hash detection

The interactions between `USE_HTTPS` and `SHA1_BACKEND` have been
streamlined. Previously we would have accepted not quite working
configurations (like, `-DUSE_HTTPS=OFF -DSHA1_BACKEND=OpenSSL`) and, as
the OpenSSL detection only ran with `USE_HTTPS`, the link would fail.

The detection was moved to a new `USE_SHA1`, modeled after `USE_HTTPS`,
which takes the values "CollisionDetection/Backend/Generic", to better
match how the "hashing backend" is selected, the default (ON) being
"CollisionDetection".

Note that, as `SHA1_BACKEND` is still used internally, you might need to
check what customization you're using it for.

1 files changed, 7 insertions, 0 deletions

diff --git a/docs/changelog.md b/docs/changelog.md
index 75f839212..e5eaf0794 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -10,6 +10,13 @@ v0.28 + 1
   system http-parser implementation despite incompatibilities, you can
   specify `-DUSE_HTTP_PARSER=system` to CMake.
 
+* The interactions between `USE_HTTPS` and `SHA1_BACKEND` have been
+  streamlined. The detection was moved to a new `USE_SHA1`, modeled after
+  `USE_HTTPS`, which takes the values "CollisionDetection/Backend/Generic", to
+  better match how the "hashing backend" is selected, the default (ON) being
+  "CollisionDetection". If you were using `SHA1_BACKEND` previously, you'll
+  need to check the value you've used, or switch to the autodetection.
+
 ### Changes or improvements
 
 * libgit2 can now correctly cope with URLs where the host contains a colon