Fix Issue dlang#686

This fixes [Issue dlang#686](dlang#686), "multiple -march with LDC".

Note that I have not run this through the full DUB test suite, as it wasn't obvious to me how to use it.

source/dub/compilers/buildsettings.d

@@ -80,9 +80,9 @@ struct BuildSettings {
 		addPostBuildCommands(bs.postBuildCommands);
 	}
 
-	void addDFlags(in string[] value...) { dflags ~= value; }
+	void addDFlags(in string[] value...) { add(dflags, value, true); }
 	void removeDFlags(in string[] value...) { remove(dflags, value); }
-	void addLFlags(in string[] value...) { lflags ~= value; }
+	void addLFlags(in string[] value...) { add(lflags, value, true); }
 	void addLibs(in string[] value...) { add(libs, value); }
 	void addLinkerFiles(in string[] value...) { add(linkerFiles, value); }
 	void addSourceFiles(in string[] value...) { add(sourceFiles, value); }