Don't use macro expressions in another macro

We use icecc to speed up compilation in our project. After this change
https://swiftshader-review.googlesource.com/12308 chromium doesn't
compile via icecc as __has_include(<atomic>) is never expanded to 0 or 1
on icecc host and file <atomic> is not being included. My guess is that
icecc host gets partially expanded code and it's not being passed by
preprocessor again so condition #if __has_include(<atomic>) (expanded
from #if USE_STD_ATOMIC) is never fulfilled.
Bug: chromium:783135
Change-Id: I09cf120c399a794a15dec05d6edd74c7848fc53a
Reviewed-on: https://swiftshader-review.googlesource.com/13588
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Cezary Kulakowski <ckulakowski@opera.com>

src/Common/Thread.hpp

diff --git a/src/Common/Thread.hpp b/src/Common/Thread.hpp
index 7b4e0ca..186d1bd 100644
--- a/src/Common/Thread.hpp
+++ b/src/Common/Thread.hpp
@@ -31,7 +31,9 @@
 #include <stdlib.h>
 
 #if defined(__clang__)
-#define USE_STD_ATOMIC __has_include(<atomic>) // clang has an explicit check for the availability of atomic
+#if __has_include(<atomic>) // clang has an explicit check for the availability of atomic
+#define USE_STD_ATOMIC 1
+#endif
 // atomic is available in C++11 or newer, and in Visual Studio 2012 or newer
 #elif (defined(_MSC_VER) && (_MSC_VER >= 1700)) || (__cplusplus >= 201103L)
 #define USE_STD_ATOMIC 1