Modifying an ESP IDF component (GNU MAKE)

[PeterR]

FIXED: Missing a menuconfig option! Doh!

Using legacy GNU make:
I understand that to modify an ESP IDF component you copy the component to your own application/components directory and then modify.
This works for me with a small example.

When I copy the modified IDF component to my large application I can see that the modified component has been compiled (I added some .c files) but the original esp-idf/components header file is being included.

What might be going wrong? EDIT: How can I check include paths?
Is there a way to just build one component? GNU make is so slow and asking the build system to build the problem component first would help speed things up.