- Detect libstdc++ from gcc-5 and never.

Author: phprus

include/ghc/filesystem.hpp

@@ -240,6 +240,9 @@
 #define GHC_HAS_STD_EXPERIMENTAL_STRING_VIEW
 #elif defined(__GNUC__) && (((__GNUC__ == 4) && (__GNUC_MINOR__ >= 9)) || (__GNUC__ > 4)) && (__cplusplus >= 201402)
 #define GHC_HAS_STD_EXPERIMENTAL_STRING_VIEW
+#elif defined(__GLIBCXX__) && defined(_GLIBCXX_USE_DUAL_ABI) && (__cplusplus >= 201402)
+// macro _GLIBCXX_USE_DUAL_ABI is always defined in libstdc++ from gcc-5 and newer
+#define GHC_HAS_STD_EXPERIMENTAL_STRING_VIEW
 #endif
 
 #if defined(GHC_HAS_STD_STRING_VIEW)