Fix usage with musl libc.

linux_syscall_support.h

 /* Copyright (c) 2005-2011, Google Inc.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 82 matching lines @@
 #ifndef SYS_CPLUSPLUS
 #ifdef __cplusplus
 /* Some system header files in older versions of gcc neglect to properly
  * handle being included from C++. As it appears to be harmless to have
  * multiple nested 'extern "C"' blocks, just add another one here.
  */
 extern "C" {
 #endif
 
 #include <errno.h>
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE /* musl libc defines loff_t only with _GNU_SOURC */

[Mark Seaborn, 2016/03/13 19:59:49]

Typo: "SOURC"

I'm not sure that defining _GNU_SOURCE here is a good idea, because:
 * it will leak to files #included after linux_syscall_support.h
 * it won't work if fcntl.h was #included earlier

What's your use case?  i.e. What are you building that uses
linux_syscall_support.h?

+#endif
 #include <fcntl.h>
 #include <sched.h>
 #include <signal.h>
 #include <stdarg.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <string.h>
 #include <sys/ptrace.h>
 #include <sys/resource.h>
 #include <sys/time.h>
@@ skipping 935 matching lines @@
 #define __NR_move_pages         239
 #endif
 /* End of aarch64 definitions                                                */
 #elif defined(__x86_64__)
 #ifndef __NR_pread64
 #define __NR_pread64             17
 #endif
 #ifndef __NR_pwrite64
 #define __NR_pwrite64            18
 #endif
+#ifndef __NR_pread
+#define __NR_pread __NR_pread64

[Mark Seaborn, 2016/03/13 19:59:49]

Please explain what this fixes.  What's your test case?

+#endif
+#ifndef __NR_pwrite
+#define __NR_pwrite __NR_pwrite64
+#endif
 #ifndef __NR_setresuid
 #define __NR_setresuid          117
 #define __NR_getresuid          118
 #define __NR_setresgid          119
 #define __NR_getresgid          120
 #endif
 #ifndef __NR_quotactl
 #define __NR_quotactl           179
 #endif
 #ifndef __NR_gettid
@@ skipping 2960 matching lines @@
 # pragma pop_macro("fstat64")
 # pragma pop_macro("lstat64")
 #endif
 
 #if defined(__cplusplus) && !defined(SYS_CPLUSPLUS)
 }
 #endif
 
 #endif
 #endif