In order to enable FPM in your PHP build you need to add --enable-fpm to your configure line.
There are several other FPM-specific configure options (all of them optional):
--with-fpm-user - set FPM user (default - nobody).
--with-fpm-group - set FPM group (default - nobody).
If you get a compile/link error with WCOREDUMP undefined, then define the following in sapi/fpm/fpm/fpm_children.c.
This happened to me when running on AIX, which apparently does not define this macro.
#define WCOREDUMP(x) 0