Move ninja requirements to a dynamic check, when actually building

It isn't practical to determine whether we'll build LLVM very early in the
pipeline, so move the ninja checking to a dynamic check.

src/bootstrap/lib.rs

@@ -650,7 +650,7 @@ impl Build {
             }
         } else {
             let base = self.llvm_out(self.config.build).join("build");
-            let base = if !self.config.ninja && self.config.build.contains("msvc") {
+            let base = if !self.ninja() && self.config.build.contains("msvc") {
                 if self.config.llvm_optimize {
                     if self.config.llvm_release_debuginfo {
                         base.join("RelWithDebInfo")
@@ -1328,6 +1328,43 @@ impl Build {
         }
         fs::remove_file(f).unwrap_or_else(|_| panic!("failed to remove {:?}", f));
     }
+
+    /// Returns if config.ninja is enabled, and checks for ninja existence,
+    /// exiting with a nicer error message if not.
+    fn ninja(&self) -> bool {
+        let mut cmd_finder = crate::sanity::Finder::new();
+
+        if self.config.ninja_in_file {
+            // Some Linux distros rename `ninja` to `ninja-build`.
+            // CMake can work with either binary name.
+            if cmd_finder.maybe_have("ninja-build").is_none()
+                && cmd_finder.maybe_have("ninja").is_none()
+            {
+                eprintln!(
+                    "
+Couldn't find required command: ninja
+You should install ninja, or set ninja=false in config.toml
+"
+                );
+                std::process::exit(1);
+            }
+        }
+
+        // If ninja isn't enabled but we're building for MSVC then we try
+        // doubly hard to enable it. It was realized in #43767 that the msbuild
+        // CMake generator for MSVC doesn't respect configuration options like
+        // disabling LLVM assertions, which can often be quite important!
+        //
+        // In these cases we automatically enable Ninja if we find it in the
+        // environment.
+        if !self.config.ninja_in_file && self.config.build.contains("msvc") {
+            if cmd_finder.maybe_have("ninja").is_some() {
+                return true;
+            }
+        }
+
+        self.config.ninja_in_file
+    }
 }
 
 #[cfg(unix)]