On GNU/Hurd, 'PATH_MAX' is undefined.  Provide a reasonable default value.
Adapted from <https://github.com/libgit2/libgit2/pull/7100>.

diff --git a/tests/clar/clar.h b/tests/clar/clar.h
index ca72292ae91..98e2b14e880 100644
--- a/t/unit-tests/clar/clar.h
+++ b/t/unit-tests/clar/clar.h
@@ -14,6 +14,8 @@
 # define CLAR_MAX_PATH 4096
 #elif defined(_WIN32)
 # define CLAR_MAX_PATH MAX_PATH
+#elif !defined(PATH_MAX)			  /* GNU/Hurd */
+# define CLAR_MAX_PATH 4096
 #else
 # define CLAR_MAX_PATH PATH_MAX
 #endif