Fix build on the GNU Hurd.

SYS_* system call numbers are not defined on the Hurd system.
Three of them, SYS_mbind, SYS_set_mempolicy, and SYS_gettid,
are used in "common_linux.h".  The first two are about setting NUMA
(Non‐Uniform Memory Access) policy.  NUMA is not supported by the Hurd
for now.  The third one refers to the Linux specific model of threads.
Therefore it is safe to simply disable these syscalls on the GNU Hurd.

Author: nigkon

common_linux.h

@@ -70,7 +70,8 @@ extern long int syscall (long int __sysno, ...);
 static inline int my_mbind(void *addr, unsigned long len, int mode,
 			   unsigned long *nodemask, unsigned long maxnode,
 			   unsigned flags) {
-#if defined (__LSB_VERSION__) || defined(ARCH_ZARCH) || defined(__PIZLONATOR_WAS_HERE__)
+#if defined (__LSB_VERSION__) || defined(ARCH_ZARCH) || defined(__PIZLONATOR_WAS_HERE__) \
+  || defined(__gnu_hurd__)
 // So far,  LSB (Linux Standard Base) don't support syscall().
 // https://lsbbugs.linuxfoundation.org/show_bug.cgi?id=3482
         return 0;
@@ -82,7 +83,7 @@ static inline int my_mbind(void *addr, unsigned long len, int mode,
 }
 
 static inline int my_set_mempolicy(int mode, const unsigned long *addr, unsigned long flag) {
-#if defined (__LSB_VERSION__) || defined(ARCH_ZARCH)
+#if defined (__LSB_VERSION__) || defined(ARCH_ZARCH) || defined(__gnu_hurd__)
 // So far,  LSB (Linux Standard Base) don't support syscall().
 // https://lsbbugs.linuxfoundation.org/show_bug.cgi?id=3482
   return 0;