bootstrap: Define CMake platform if DragonFly.

CMAKE_SYSTEM_NAME is defined on a cross build if the target is
recognized.  Without this explicit definition cmake will assume that
we're building for the host platform which can bring in unwanted
compiler and linker flags.

Also, add a warning on cross builds with unknown target to aid in
cross builds for future platforms.

Author: Alex Zepeda

src/bootstrap/download-ci-llvm-stamp

@@ -1,4 +1,4 @@
 Change this file to make users of the `download-ci-llvm` configuration download
 a new version of LLVM from CI, even if the LLVM submodule hasn’t changed.
 
-Last change is for: https://github.com/rust-lang/rust/pull/112931
+Last change is for: https://github.com/rust-lang/rust/pull/113996

src/bootstrap/llvm.rs

@@ -559,6 +559,8 @@ fn configure_cmake(
 
         if target.contains("netbsd") {
             cfg.define("CMAKE_SYSTEM_NAME", "NetBSD");
+        } else if target.contains("dragonfly") {
+            cfg.define("CMAKE_SYSTEM_NAME", "DragonFly");
         } else if target.contains("freebsd") {
             cfg.define("CMAKE_SYSTEM_NAME", "FreeBSD");
         } else if target.contains("windows") {
@@ -569,7 +571,12 @@ fn configure_cmake(
             cfg.define("CMAKE_SYSTEM_NAME", "SunOS");
         } else if target.contains("linux") {
             cfg.define("CMAKE_SYSTEM_NAME", "Linux");
+        } else {
+            builder.info(
+                "could not determine CMAKE_SYSTEM_NAME from the target `{target}`, build may fail",
+            );
         }
+
         // When cross-compiling we should also set CMAKE_SYSTEM_VERSION, but in
         // that case like CMake we cannot easily determine system version either.
         //