Summary: support gcc atomic ops on clang too

Clang actually does support __atomic_XXX atomic ops builtins but does
not advertise itselt as gcc 4.7 or later. So we now detect clang
separetely.

We're enabling gcc atomic ops on clang >= 3.4 since this is the oldest
version that I can test.

This should fix issue #797.

src/base/atomicops.h

@@ -102,8 +102,14 @@
                      + __GNUC_MINOR__ * 100           \
                      + __GNUC_PATCHLEVEL__)
 
+#define CLANG_VERSION (__clang_major__ * 10000         \
+                       + __clang_minor__ * 100         \
+                       + __clang_patchlevel__)
+
 #if defined(TCMALLOC_PREFER_GCC_ATOMICS) && defined(__GNUC__) && GCC_VERSION >= 40700
 #include "base/atomicops-internals-gcc.h"
+#elif defined(TCMALLOC_PREFER_GCC_ATOMICS) && defined(__clang__) && CLANG_VERSION >= 30400
+#include "base/atomicops-internals-gcc.h"
 #elif defined(__MACH__) && defined(__APPLE__)
 #include "base/atomicops-internals-macosx.h"
 #elif defined(__GNUC__) && defined(ARMV6)
@@ -120,6 +126,8 @@
 #include "base/atomicops-internals-mips.h"
 #elif defined(__GNUC__) && GCC_VERSION >= 40700
 #include "base/atomicops-internals-gcc.h"
+#elif defined(__clang__) && CLANG_VERSION >= 30400
+#include "base/atomicops-internals-gcc.h"
 #else
 #error You need to implement atomic operations for this architecture
 #endif