Fix build with gcc (#27)

Summary:
kDescriptor needs full qualification because gcc complained of a
mismatched declaration and definition otherwise.

SimpleFixedString conditions were reordered because
`#if defined(foo) && foo(bar)` is a preprocessor syntax error with gcc.
Pull Request resolved: facebookincubator/fbjni#27

Reviewed By: swolchok

Differential Revision: D19233972

Pulled By: dreiss

fbshipit-source-id: 60a2f1a9aa96ff4c35915e045481835041245c1f

first-party/fbjni/cxx/fbjni/detail/Meta.h

@@ -294,11 +294,11 @@ struct jtype_traits {
  public:
   // The jni type signature (described at
   // http://docs.oracle.com/javase/1.5.0/docs/guide/jni/spec/types.html).
-  static constexpr decltype(descriptor()) /* detail::SimpleFixedString<_> */ kDescriptor = descriptor();
+  static constexpr decltype(jtype_traits<T>::descriptor()) /* detail::SimpleFixedString<_> */ kDescriptor = descriptor();
 
   // The signature used for class lookups. See
   // http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#getName().
-  static constexpr decltype(base_name()) /* detail::SimpleFixedString<_> */ kBaseName = base_name();
+  static constexpr decltype(jtype_traits<T>::base_name()) /* detail::SimpleFixedString<_> */ kBaseName = base_name();
 };
 
 template <typename T>

first-party/fbjni/cxx/fbjni/detail/SimpleFixedString.h

@@ -19,10 +19,17 @@
 #pragma once
 
 #include <cassert>
+#include <cstring>
 #include <stdexcept>
 #include <string>
 #include <utility>
 
+#if defined(__has_feature)
+#define FBJNI_HAS_FEATURE(...) __has_feature(__VA_ARGS__)
+#else
+#define FBJNI_HAS_FEATURE(...) 0
+#endif
+
 namespace facebook {
 namespace jni {
 
@@ -50,7 +57,7 @@ static_assert(
     "Someone appears to have broken constexpr_strlen...");
 
 constexpr size_t constexpr_strlen(const char* s) {
-#if defined(__has_feature) && __has_feature(cxx_constexpr_string_builtins)
+#if FBJNI_HAS_FEATURE(cxx_constexpr_string_builtins)
   // clang provides a constexpr builtin
   return __builtin_strlen(s);
 #elif defined(__GNUC__) && !defined(__clang__)