_Actually_ update ab.

Author: davidgiven

build/ab.mk

@@ -114,5 +114,6 @@ $(OBJ)/build.ninja $(OBJ)/build.targets &:
 
 include $(OBJ)/build.targets
 .PHONY: $(ninja-targets)
-$(ninja-targets) : $(OBJ)/build.ninja
+.NOTPARALLEL:
+$(ninja-targets): $(OBJ)/build.ninja
 	+$(hide) $(NINJA) -f $(OBJ)/build.ninja $@