Use _LIBCPP_VERSION macro instead of __clang (#531)

src/util/Generator.h

@@ -5,22 +5,22 @@
 #ifndef CPPCORO_GENERATOR_HPP_INCLUDED
 #define CPPCORO_GENERATOR_HPP_INCLUDED
 
-// coroutines are still experimental in clang, adapt the appropriate
-// namespaces.
-#ifdef __clang__
+#include <exception>
+#include <functional>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+
+// coroutines are still experimental in clang libcpp,
+// adapt the appropriate namespaces.
+#ifdef _LIBCPP_VERSION
 #include <experimental/coroutine>
 namespace qlever_stdOrExp = std::experimental;
 #else
 #include <coroutine>
 namespace qlever_stdOrExp = std;
 #endif
 
-#include <exception>
-#include <functional>
-#include <iterator>
-#include <type_traits>
-#include <utility>
-
 namespace cppcoro {
 template <typename T>
 class generator;

src/util/streamable_generator.h

@@ -7,17 +7,18 @@
 #include <exception>
 #include <sstream>
 
-#include "./ostringstream.h"
-
-// coroutines are still experimental in clang, adapt the appropriate namespaces.
-#ifdef __clang__
+// coroutines are still experimental in clang libcpp,
+// adapt the appropriate namespaces.
+#ifdef _LIBCPP_VERSION
 #include <experimental/coroutine>
 namespace qlever_stdOrExp = std::experimental;
 #else
 #include <coroutine>
 namespace qlever_stdOrExp = std;
 #endif
 
+#include "./ostringstream.h"
+
 namespace ad_utility::stream_generator {
 
 /**