Use BOOST_GCC, defined in config.hpp only for real (non-Clang-emulated) GCC

builds, in place of relevant uses of the __GNUC__ macro.

Author: tzlaine

include/boost/stl_interfaces/fwd.hpp

@@ -17,14 +17,14 @@
 
 #ifndef BOOST_STL_INTERFACES_DOXYGEN
 
-#if defined(_MSC_VER) || defined(__GNUC__) && __GNUC__ < 8
+#if defined(_MSC_VER) || defined(BOOST_GCC) && __GNUC__ < 8
 #define BOOST_STL_INTERFACES_NO_HIDDEN_FRIEND_CONSTEXPR
 #define BOOST_STL_INTERFACES_HIDDEN_FRIEND_CONSTEXPR
 #else
 #define BOOST_STL_INTERFACES_HIDDEN_FRIEND_CONSTEXPR constexpr
 #endif
 
-#if defined(__GNUC__) && __GNUC__ < 9
+#if defined(BOOST_GCC) && __GNUC__ < 9
 #define BOOST_STL_INTERFACES_CONCEPT concept bool
 #else
 #define BOOST_STL_INTERFACES_CONCEPT concept

include/boost/stl_interfaces/view_adaptor.hpp

@@ -24,7 +24,7 @@
 #endif
 
 #if !BOOST_STL_INTERFACES_USE_CPP23_STD_RANGE_ADAPTOR_CLOSURE &&               \
-    BOOST_STL_INTERFACES_USE_CONCEPTS && defined(__GNUC__) && 12 <= __GNUC__
+    BOOST_STL_INTERFACES_USE_CONCEPTS && defined(BOOST_GCC) && 12 <= __GNUC__
 #define BOOST_STL_INTERFACES_USE_LIBSTDCPP_GCC12_RANGE_ADAPTOR_CLOSURE 1
 #else
 #define BOOST_STL_INTERFACES_USE_LIBSTDCPP_GCC12_RANGE_ADAPTOR_CLOSURE 0