bug#20536: avoid new gcc warning: ENOTSUP vs EOPNOTSUPP

[Jim Meyering]

Building with very new gcc-from-git, I encountered 3 new warnings.
Here's a patch to address them:

Without this change, very recent gcc (e.g., version 6.0.0 20150509)
would print the following when configured with --enable-gcc-warnings:

  src/copy.c:165:30: error: logical 'or' of equal expressions \
    [-Werror=logical-op]
    && (errno == EOPNOTSUPP || errno == ENOTSUP || errno == ENOSYS))
                           ^
* src/system.h (is_ENOTSUP): New function.
* src/copy.c (punch_hole): Use it.
* src/ls.c (errno_unsupported, gobble_file): Use it.

0001-build-avoid-a-warning-form-gcc-s-new-Wlogical-op.patch
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