Support self-contained built ?

[jwfang]

i want to build a "re-producable" FFmpeg and all its dependency 3rd party libraries in a directory without any dependency on the host system.
But FFmpeg include x264 with #include <x264.h>, and the configure script APPEND -I flags. So if i have a x264 installed in /usr/local, it will be used, and there is no means to force FFmpeg to use my own libx264.

I can think of the following solution:
1) if --extra-cflags PREPEND instead of append
2) --with-foolib=<dir>
3) include with "" instead of <>, and use GCC's -iquote

[jwfang]

sorry, i use 1) --extra-cflags solved this.
the options are processed before all config tests which add my locally installed x264.