avoid a rare race in the "build" target

when two Make instances run in parallel and the timing is just right for
one of them to create the dir after the other checked for its existence,
but before it tries to create it

makefiles/targets.mk

@@ -14,7 +14,7 @@ $(SILENT_TARGET_PREFIX).SILENT:
 
 # dir
 build:
-	mkdir $@
+	mkdir -p $@
 
 sanity-checks:
 	which $(CC) &>/dev/null || { echo "C compiler ($(CC)) not installed. Aborting."; exit 1; }