fix -latomic test for complex cases

Author: jblomer

cmake/modules/SearchInstalledSoftware.cmake

@@ -2184,13 +2184,16 @@ endif()
 check_cxx_source_compiles("
 #include <atomic>
 #include <cstdint>
+struct S { int a[100]; };
 int main() {
    std::atomic<int> a1;
    int a1val = a1.load();
    (void)a1val;
    std::atomic<uint64_t> a2;
    uint64_t a2val = a2.load(std::memory_order_relaxed);
    (void)a2val;
+   std::atomic<S> s;
+   (void)s.is_lock_free();
    return 0;
 }
 " ROOT_HAVE_CXX_ATOMICS_WITHOUT_LIB)