properly detect GCC atomics

I was naive. GCC atomics were added in 4.1.2, and not easily detectable
without configure tests. 32bit platforms, centos5, etc.

configure.ac

@@ -454,6 +454,20 @@ fi
 
 AC_C_ALIGNMENT
 
+dnl Check for our specific usage of GCC atomics.
+dnl These were added in 4.1.2, but 32bit OS's may lack shorts and 4.1.2
+dnl lacks testable defines.
+have_gcc_atomics=no
+AC_MSG_CHECKING(for GCC atomics)
+AC_TRY_LINK([],[
+  unsigned short a;
+  unsigned short b;
+  b = __sync_add_and_fetch(&a, 1);
+  b = __sync_sub_and_fetch(&a, 2);
+  ],[have_gcc_atomics=yes
+  AC_DEFINE(HAVE_GCC_ATOMICS, 1, [GCC Atomics available])])
+AC_MSG_RESULT($have_gcc_atomics)
+
 dnl Check for the requirements for running memcached with less privileges
 dnl than the default privilege set. On Solaris we need setppriv and priv.h
 dnl If you want to add support for other platforms you should check for

thread.c

@@ -43,7 +43,7 @@ pthread_mutex_t cache_lock;
 /* Connection lock around accepting new connections */
 pthread_mutex_t conn_lock = PTHREAD_MUTEX_INITIALIZER;
 
-#if !defined(__GNUC__) && !defined(__sun)
+#if !defined(HAVE_GCC_ATOMICS) && !defined(__sun)
 pthread_mutex_t atomics_mutex = PTHREAD_MUTEX_INITIALIZER;
 #endif
 
@@ -79,29 +79,29 @@ static pthread_cond_t init_cond;
 static void thread_libevent_process(int fd, short which, void *arg);
 
 inline unsigned short refcount_incr(unsigned short *refcount) {
-#ifdef __GNUC__
+#ifdef HAVE_GCC_ATOMICS
     return __sync_add_and_fetch(refcount, 1);
 #elif defined(__sun)
     return atomic_inc_ushort_nv(refcount);
 #else
     unsigned short res;
     mutex_lock(&atomics_mutex);
-    *refcount++;
+    (*refcount)++;
     res = *refcount;
     pthread_mutex_unlock(&atomics_mutex);
     return res;
 #endif
 }
 
 inline unsigned short refcount_decr(unsigned short *refcount) {
-#ifdef __GNUC__
+#ifdef HAVE_GCC_ATOMICS
     return __sync_sub_and_fetch(refcount, 1);
 #elif defined(__sun)
     return atomic_dec_ushort_nv(refcount);
 #else
     unsigned short res;
     mutex_lock(&atomics_mutex);
-    *refcount--;
+    (*refcount)--;
     res = *refcount;
     pthread_mutex_unlock(&atomics_mutex);
     return res;