Only look for old Linux kernel versions on Linux.

This do not work well on GNU Hurd, where the kernel version is 0.9.

This get test_malloc_compliance working on GNU Hurd.

This fix is related to #2027.

Author: Petter Reinholdtsen

test/common/memory_usage.h

@@ -74,7 +74,8 @@ namespace utils {
         peakUsage
     };
 
-#if __unix__
+#if __linux__
+
     inline unsigned LinuxKernelVersion()
     {
         unsigned digit1, digit2, digit3;
@@ -125,8 +126,11 @@ namespace utils {
                 break;
             }
         }
-        // VmPeak is available in kernels staring 2.6.15
-        if (stat != peakUsage || LinuxKernelVersion() >= 2006015)
+        // VmPeak is available in Linux kernels staring 2.6.15
+        if (stat != peakUsage
+#if __linux__
+            || LinuxKernelVersion() >= 2006015)
+#endif
             ASSERT(size, "Invalid /proc/self/status format, pattern not found.");
         fclose(fst);
         return size * 1024;