HFS+, the FS on Darwin, is case insensitive. So this patch adjusts
filename_cmp.c to ignore the casing when comparing filenames on Darwin.

include/ChangeLog:

        * filenames.h (HAVE_CASE_INSENSITIVE_FILE_SYSTEM): Define
        on Darwin, as well as on the systems that use a DOS-like
        filesystem.

libiberty/ChangeLog:

        * filename_cmp.c (filename_cmp, filename_ncmp): Add handling of
        HAVE_CASE_INSENSITIVE_FILE_SYSTEM.

From-SVN: r175762