[flang] Tweak the conditions for the GCC 7/libstdc++ workaround

[mstorsjo]

Summary

This adjusts the workaround from D104731.

The issue lies in libstdc++'s classes, not GCC itself, and manifests
itself in the same way if building e.g. with clang while using
libstdc++ headers from GCC 7 (e.g. if building with Clang on Ubuntu 18.04,
while using the system default C++ library).

Therefore, change the condition to look for the version of libstdc++
instead of the compiler.

[klausler]

Thanks for improving this patch.

flang/include/flang/Evaluate/type.h

145

You don't need to check whether _GLIBCXX_RELEASE is defined; if it is not, it'll be replaced with 0 and the test will fail anyway.

225

Same here.

[mstorsjo]

flang/include/flang/Evaluate/type.h

145

Ok, although leaving out that check cause warnings if building with -Wundef.

[klausler]

flang/include/flang/Evaluate/type.h

145

Then please leave the check in place; we build f18 with -Werror.

flang/include/flang/Evaluate/type.h

public:
   constexpr int kind() const {
     CHECK(kind_ > 0);
     return kind_;
   }
   constexpr const semantics::ParamValue *charLengthParamValue() const {
     return charLengthParamValue_;
   }
   constexpr std::optional<std::int64_t> knownLength() const {
-#if !__clang__ && __GNUC__ == 7
+#if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 7
     if (knownLength_ < 0) {
       return std::nullopt;
     }
 #endif
     return knownLength_;
   }
   std::optional<Expr<SubscriptInteger>> GetCharLength() const;
 
enum SpecialKind {
     AssumedTypeKind = -3, // TYPE(*)
   };
 
   constexpr DynamicType() {}
 
   TypeCategory category_{TypeCategory::Derived}; // overridable default
   int kind_{0};
   const semantics::ParamValue *charLengthParamValue_{nullptr};
-#if !__clang__ && __GNUC__ == 7
+#if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 7
   // GCC 7's optional<> lacks a constexpr operator=
   std::int64_t knownLength_{-1};
 #else
   std::optional<std::int64_t> knownLength_;
 #endif
   const semantics::DerivedTypeSpec *derived_{nullptr}; // TYPE(T), CLASS(T)
 };