Fixes some issues with C++11.

In compiler.h, it adds the missing "std::" namespace prefixes to the
memory_barrier() definition.

In gpsd.h it:

1) Moves the include of compiler.h outside the conditional 'extern
"C"', since the "atomic" stuff in the former is incompatible with the
latter.

2) Fixes DEVICEHOOKPATH for C++11 (C++11 requires spaces between
literals and string macros).

3) Cleans up some ordering of system includes left over from the
former head/tail setup.

TESTED:
Ran "scons build-all check" on OSX 10.5-10.12, Ubuntu 14, CentOS 7,
Fedora 25, FreeBSD 10.3, OpenBSD 5.6 (32- and 64-bit), and NetBSD
6.1.5.  Also tested Qt builds with OSX 10.9 (Qt4 and Qt5), OSX 10.12
(Qt5), and Fedora 25 (Qt5).
Observed the correct DEVICEHOOKPATH in the log on OSX.

compiler.h

@@ -90,7 +90,7 @@ static inline void memory_barrier(void)
   /* we are C++ */
   #if __cplusplus >= 201103L
     /* C++11 and later has atomics, earlier do not */
-    atomic_thread_fence(memory_order_seq_cst);
+    std::atomic_thread_fence(std::memory_order_seq_cst);
   #endif
 #elif defined HAVE_STDATOMIC_H
     /* we are C and atomics are in C98 and newer */

gpsd.h

@@ -7,22 +7,21 @@
 #ifndef _GPSD_H_
 #define _GPSD_H_
 
+#include "compiler.h"	/* Must be outside extern "C" for "atomic" */
+
 # ifdef __cplusplus
 extern "C" {
 # endif
 
+#include <stdarg.h>
 #include <stdbool.h>
-#include <stdio.h>
-#include <time.h>    /* for time_t */
-
-#include "gpsd_config.h"
-
 #include <stdint.h>
-#include <stdarg.h>
+#include <stdio.h>
 #include <termios.h>
+#include <time.h>    /* for time_t */
 
 #include "gps.h"
-#include "compiler.h"
+#include "gpsd_config.h"
 #include "os_compat.h"
 
 /*
@@ -991,7 +990,7 @@ PRINTF_FUNC(3, 4) void gpsd_log(const struct gpsd_errout_t *, const int, const c
 void cfmakeraw(struct termios *);
 #endif /* defined(__CYGWIN__) */
 
-#define DEVICEHOOKPATH "/"SYSCONFDIR"/gpsd/device-hook"
+#define DEVICEHOOKPATH "/" SYSCONFDIR "/gpsd/device-hook"
 
 # ifdef __cplusplus
 }