On various BSD's, fileno(fp) is implemented as a macro that directly
accesses the fields in the FILE * object, which breaks a function that
accepts a "void *fp" parameter and calls fileno(fp) and expect it to
work.

Work it around by adding a compile-time knob FILENO_IS_A_MACRO that
inserts a real helper function in the middle of the callchain.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>