backport https://github.com/x42/gmsynth.lv2/commit/13acf5e
see https://bugs.gentoo.org/982836
Ensure that $(BUILDDIR) is created before using it
--- a/Makefile
+++ b/Makefile
@@ -128,6 +128,7 @@ $(BUILDDIR)$(LV2NAME).ttl: Makefile lv2ttl/$(LV2NAME).*.in
 		lv2ttl/$(LV2NAME).ttl.in > $(BUILDDIR)$(LV2NAME).ttl
 
 $(BUILDDIR)%.sf2: sf2/*.sf2
+	@mkdir -p $(BUILDDIR)
 	cp -v sf2/$(*F).sf2 $@
 
 FLUID_SRC = \