configure: standardize handling of 64 bit atomics

replace the custom QT_NO_STD_ATOMIC64 with a regular public feature, and
give libatomic an empty source rather than using a separate config test.

Change-Id: Iaf4a7f4c4874f61bf93aa58fe41843a86baf1ab7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/arch/arch.pri

@@ -4,4 +4,4 @@ HEADERS += \
     arch/qatomic_bootstrap.h \
     arch/qatomic_cxx11.h
 
-qtConfig(libatomic): QMAKE_USE += libatomic
+qtConfig(std-atomic64): QMAKE_USE += libatomic

src/corelib/arch/qatomic_cxx11.h

@@ -187,7 +187,7 @@ template <> Q_DECL_CONSTEXPR inline bool QAtomicTraits<2>::isLockFree()
 { return false; }
 #endif
 
-#ifndef QT_NO_STD_ATOMIC64
+#if QT_CONFIG(std_atomic64)
 template<> struct QAtomicOpsSupport<8> { enum { IsSupported = 1 }; };
 #  define Q_ATOMIC_INT64_IS_SUPPORTED
 #  if ATOMIC_LLONG_LOCK_FREE == 2

src/corelib/configure.json

@@ -69,9 +69,10 @@
             ]
         },
         "libatomic": {
-            "label": "64 bit atomics in libatomic",
+            "label": "64 bit atomics",
             "test": "common/atomic64",
             "sources": [
+                "",
                 "-latomic"
             ]
         },
@@ -116,11 +117,6 @@
     },
 
     "tests": {
-        "atomic64": {
-            "label": "64 bit atomics",
-            "type": "compile",
-            "test": "common/atomic64"
-        },
         "atomicfptr": {
             "label": "working std::atomic for function pointers",
             "type": "compile",
@@ -284,13 +280,8 @@
         },
         "std-atomic64": {
             "label": "64 bit atomic operations",
-            "condition": "tests.atomic64 || libs.libatomic",
-            "output": [ { "type": "define", "negative": true, "name": "QT_NO_STD_ATOMIC64" } ]
-        },
-        "libatomic": {
-            "label": "64 bit atomic operations in libatomic",
-            "condition": "!tests.atomic64 && libs.libatomic",
-            "output": [ "privateFeature" ]
+            "condition": "libs.libatomic",
+            "output": [ "publicFeature" ]
         },
         "mimetype": {
             "label": "Mimetype handling",