linkat: Make implementation robust against system behaviour variations.

linkat: Make implementation robust against system behaviour variations.

* m4/link-follow.m4 (gl_FUNC_LINK_FOLLOWS_SYMLINK): Define
LINK_FOLLOWS_SYMLINKS to -1 if it needs a runtime test in the Solaris
way, and to -2 if it needs a generic runtime test.
* lib/linkat.c (solaris_optimized_link_immediate,
solaris_optimized_link_follow): New functions.
* tests/test-linkat.c (EXPECT_LINK_HARDLINKS_SYMLINKS): New macro.
(check_same_link): Use it.