[libiberty/filename_cmp] Darwin has case-insensitive filesystems

[libiberty/filename_cmp] Darwin has case-insensitive filesystems

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.