strtoull: Work around a bug on native Windows and Minix. * lib/stdlib.in.h (strtoull): Override if REPLACE_STRTOULL is 1. * m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Initialize REPLACE_STRTOULL. * m4/strtoull.m4 (gl_FUNC_STRTOULL): Test whether strtoull works. Set REPLACE_STRTOULL. * modules/stdlib (Makefile.am): Substitute REPLACE_STRTOULL. * modules/strtoull (configure.ac): Test REPLACE_STRTOULL. * tests/test-strtoull.c (main): Add tests of hexadecimal integer syntax. * doc/posix-functions/strtoull.texi: Mention the bug.