Custom makefile

[cedyouyou]

I've already seen such a subject in this forum, but it seems no one has any answer to the problem.
I want to compile libavcodec library using a custom makefile. This makefile is readable on mingwin environment and well launchs compilation.
I've check Use custom makefile in the project menu, the makefile exists (at the same root than libavcodec.cbp), But then the 'Build method' is still disabled (stay in gray color).
Is it because of a bad format of my Makefile ? (i use msys make.exe)

Thanks,

Ced.