linkat: don't unconditionally replace on GNU/Linux
* m4/linkat.m4 (gl_FUNC_LINKAT): The compile check for AT_SYMLINK_FOLLOW was redundant for a few reasons. It was present to support compiling on new systems but running on the old narrow window of Linux 2.6.1. It setup and cleaned up test files which weren't actually used. On non __linux__ it compile tested AT_SYMLINK_FOLLOW, but that is implicit in the following check.
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