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

acinclude.m4

@@ -1559,11 +1559,11 @@ _EOF
 ])
 
 
-dnl CURL_GENERATE_BUILDINFO_TXT
+dnl CURL_PREPARE_BUILDINFO
 dnl -------------------------------------------------
 dnl Save build info for test runner to pick up and log
 
-AC_DEFUN([CURL_GENERATE_BUILDINFO_TXT], [
+AC_DEFUN([CURL_PREPARE_BUILDINFO], [
   curl_pflags=""
   case $host in
     *-apple-*) curl_pflags="${curl_pflags} APPLE";;
@@ -1592,22 +1592,19 @@ AC_DEFUN([CURL_GENERATE_BUILDINFO_TXT], [
     curl_pflags="${curl_pflags} CROSS"
   fi
   squeeze curl_pflags
-  cat >./tests/buildinfo.txt <<_EOF
-[@%:@] This is a generated file.  Do not edit.
-configure.tool: configure
-configure.args: $ac_configure_args
-host: $build
-host.os: $build_os
-host.cpu: $build_cpu
-host.vendor: $build_vendor
-target: $host
-target.os: $host_os
-target.cpu: $host_cpu
-target.vendor: $host_vendor
-target.flags: $curl_pflags
-compiler: $compiler_id
-compiler.version: $compiler_num
-_EOF
+  curl_buildinfo="
+buildinfo.configure.tool: configure
+buildinfo.configure.args: $ac_configure_args
+buildinfo.host: $build
+buildinfo.host.cpu: $build_cpu
+buildinfo.host.os: $build_os
+buildinfo.target: $host
+buildinfo.target.cpu: $host_cpu
+buildinfo.target.os: $host_os
+buildinfo.target.flags: $curl_pflags
+buildinfo.compiler: $compiler_id
+buildinfo.compiler.version: $compiler_ver
+buildinfo.sysroot: $lt_sysroot"
 ])