BLD: fix Bento build.

Issue was introduced with PR-3524, the ufunc override stuff.
BLD: add new test extensions (rational, struct_ufunc, operand_flag) to Bento build.
DOC: [bento] document that adding optional GCC performance improvements is TODO

[ci skip]
Merge pull request #3766 from rgommers/fix-bento-build

Fix bento build