build: show llvm/clang in platform flags and buildinfo.txt

Show these flags:
- `LLVM-CLANG` for mainline llvm/clang.
- `APPLE-CLANG` for Apple clang.
- `CLANG-CL` for clang-cl. (cmake only)

Also:
- GHA/linux: fix a job to build with clang, to match its descriptions.

Closes #18645

.github/workflows/linux.yml

@@ -179,6 +179,7 @@ jobs:
           - name: 'openssl clang krb5 LTO'
             install_packages: zlib1g-dev libkrb5-dev clang
             install_steps: skiprun
+            CC: clang
             generate: -DCURL_USE_OPENSSL=ON -DCURL_USE_GSSAPI=ON -DENABLE_DEBUG=ON -DCURL_LTO=ON
 
           - name: 'openssl !ipv6 !--libcurl !--digest-auth'

CMakeLists.txt

@@ -148,6 +148,15 @@ endif()
 if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
   string(APPEND _target_flags " GCC")
 endif()
+if(CMAKE_C_COMPILER_ID MATCHES "Clang")
+  if(CMAKE_C_COMPILER_ID STREQUAL "AppleClang")
+    string(APPEND _target_flags " APPLE-CLANG")
+  elseif(MSVC)
+    string(APPEND _target_flags " CLANG-CL")
+  else()
+    string(APPEND _target_flags " LLVM-CLANG")
+  endif()
+endif()
 if(MINGW)
   string(APPEND _target_flags " MINGW")
 endif()

acinclude.m4

@@ -1492,6 +1492,11 @@ AC_DEFUN([CURL_PREPARE_BUILDINFO], [
   if test "x$compiler_id" = 'xGNU_C'; then
     curl_pflags="${curl_pflags} GCC"
   fi
+  if "$compiler_id" = "APPLECLANG"; then
+    curl_pflags="${curl_pflags} APPLE-CLANG"
+  elif test "$compiler_id" = "CLANG"; then
+    curl_pflags="${curl_pflags} LLVM-CLANG"
+  fi
   case $host_os in
     mingw*) curl_pflags="${curl_pflags} MINGW";;
   esac