configure.ac: fix botched FREAD_READS_DIRECTORIES check

configure.ac: fix botched FREAD_READS_DIRECTORIES check

3adf9fdecf (configure.ac: loosen FREAD_READS_DIRECTORIES test program,
2017-06-14) broke the test program for the FREAD_READS_DIRECTORIES check
by making it syntactically invalid (a dangling ")") and by botching the
type returned from 'main' (a FILE* rather than int). As a consequence,
the test program won't even compile, thus the check fails
unconditionally. Fix these problems.

Reported-by: Jonathan Primrose <jprimros@gmail.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>