Use a custom variable in the upest level main.mk

[dolanor]

I want to share a variable containing the target dir for lib.

the tree is
/

• bin/
• lib/
• src/
  • libmylib/
  • libmyotherlib/

I want to have some variable defined in /main.mk like LIB_TARGET_DIR that I want to use in my /src/libmylib/main.mk like
TARGET_DIR := $(LIB_TARGET_DIR)
So that I can use it in /src/libmyotherlib/main.mk too
FOr 2 libs, it is not a big deal to customize the TARGET_DIR for each, but when you might have more, it more problematic.
Is there already a way to do that ? Or do I use it badly ?