run.d: Recompile dmd if necessary

Call `build.d` during startup to ensure that the tested compiler exists
and is up to date.

test/run.d

@@ -213,9 +213,20 @@ Options:
 /// Verify that the compiler has been built.
 void verifyCompilerExists(string[string] env)
 {
-    if (!env["DMD"].exists)
+    enum BUILD_D = buildPath(scriptDir, "..", "src", "build.d");
+    const scope cmd = [ `rdmd`, BUILD_D ];
+
+    // Only forward relevant flags and translate if necessary
+    const scope buildEnv = [
+        "BUILD": env["BUILD"],
+        "MODEL": env["DMD_MODEL"], // MODEL refers to compiled tests
+    ];
+
+    log("run: %-(%s %)", cmd);
+
+    if (spawnProcess(cmd, buildEnv).wait())
     {
-        stderr.writefln("%s doesn't exist, try building dmd with:\nmake -fposix.mak -j8 -C%s", env["DMD"], scriptDir.dirName.relativePath);
+        log("Failed to build dmd");
         quitSilently(1);
     }
 }