build: buildinfo.txt improvements

- cmake: drop `configure.os`.
  This also includes OS version, but thus far it's not important enough
  to include it.
- autotools: drop redundant, autotools-only `{target|host}.vendor`.
  (it's part of the triplet in `{target|host}`.)
- swap order to `*.cpu` -> `*.os` to match triplet-order.
- cmake: drop redundant `target`.
  It's manually filled and only in a (so far) few CI jobs. Let's revisit
  when this becomes useful.
- move `buildinfo.txt` to build root.
- dist: add `buildinfo.txt` to `DISTCLEANFILES`.
- autotools: detect human readable compiler version.
- autotools: replace `XXYY` `compiler.version` with "X.Y"-style.
  (also to match cmake.)
- autotools: use distinct `compiler_id` for Apple clang: `APPLECLANG`.
  To match cmake and also because the the "X.Y"-style version number
  is the Apple version, while `XXYY` was a value roughly translated to
  mainline llvm/clang version.
- show buildinfo at the end of the configure stage, when run in CI, or
  when `CURL_BUILDINFO` or `CURL_CI` env is set.

Follow-up to 1fdea16846 #14802
Assisted-by: Dan Fandrich
Ref: https://github.com/curl/curl/pull/14802#issuecomment-2334942991
Closes #14822

Makefile.am

@@ -91,6 +91,8 @@ CLEANFILES = $(VC14_LIBVCXPROJ) $(VC14_SRCVCXPROJ) \
  $(VC14_20_LIBVCXPROJ) $(VC14_20_SRCVCXPROJ)       \
  $(VC14_30_LIBVCXPROJ) $(VC14_30_SRCVCXPROJ)
 
+DISTCLEANFILES = buildinfo.txt
+
 bin_SCRIPTS = curl-config
 
 SUBDIRS = lib docs src scripts