library: dont use vm_min_free on non-Linux systems

For a specific slice of kernel versions we can get a better
estimate of the available memory before the "real" available
figure appears around kernel 3.2

However, that middle method requires a /proc/sys/vm directory.
FreeBSD 9.x used to emulate a < 2.6.27 kernel procfs meaning
it never tried the middle method. FreeBSD 10.x emulates something
more modern, but without the available figure and without a
/proc/sys/vm, so the library falls into a hole.

Hurd may to one day have this bug so we'll exclude him as well
as its triggered by whatever number appears in
/proc/sys/kernel/osrelease

References:
 commit 3f3b1a59ad
 https://bugs.debian.org/831396

proc/sysinfo.c

@@ -719,6 +719,7 @@ nextline:
 
   /* zero? might need fallback for 2.6.27 <= kernel <? 3.14 */
   if (!kb_main_available) {
+#ifdef __linux__
     if (linux_version_code < LINUX_VERSION(2, 6, 27))
       kb_main_available = kb_main_free;
     else {
@@ -734,6 +735,9 @@ nextline:
       if (mem_available < 0) mem_available = 0;
       kb_main_available = (unsigned long)mem_available;
     }
+#else
+      kb_main_available = kb_main_free;
+#endif /* linux */
   }
 }