Use default cpp fmt build script.

vendor/Fmt/doc/bootstrap/mixins/size.less

@@ -0,0 +1,10 @@
+// Sizing shortcuts
+
+.size(@width; @height) {
+  width: @width;
+  height: @height;
+}
+
+.square(@size) {
+  .size(@size; @size);
+}