[C++] Allow using symbol visibility annotations also on non-Windows platforms

In GCC and clang the symbol visibility behavior of MSVC can be mirror using `-fvisibility=hidden`.
This allows to more easily test that symbol visbility annotations work correctly and can potentially lead to smaller binaries.
The default behavior on non-Windows platforms is not changed with this commit.

See also https://gcc.gnu.org/wiki/Visibility

Also fixes a tiny mistake where the _WIN32 macro was used to silence a MSVC warning where the _MSC_VER macro should have been used instead.

Author: stephanlachnit

lang/c++/impl/json/JsonDom.hh

@@ -31,7 +31,7 @@
 
 namespace avro {
 
-class AVRO_DECL InputStream;
+class InputStream;
 
 namespace json {
 class Entity;

lang/c++/include/avro/Compiler.hh

@@ -26,15 +26,15 @@
 
 namespace avro {
 
-class AVRO_DECL InputStream;
+class InputStream;
 
 /// This class is used to implement an avro spec parser using a flex/bison
 /// compiler.  In order for the lexer to be reentrant, this class provides a
 /// lexer object for each parse.  The bison parser also uses this class to
 /// build up an avro parse tree as the avro spec is parsed.
 
-class AVRO_DECL Name;
-class AVRO_DECL ValidSchema;
+class Name;
+class ValidSchema;
 
 /// Given a stream containing a JSON schema, compiles the schema to a
 /// ValidSchema object.  Throws if the schema cannot be compiled to a valid
@@ -60,7 +60,7 @@ AVRO_DECL ValidSchema compileJsonSchemaFromString(const std::string &input);
 
 AVRO_DECL ValidSchema compileJsonSchemaFromFile(const char *filename);
 
-AVRO_DECL ValidSchema compileJsonSchemaWithNamedReferences(std::istream &is, 
+AVRO_DECL ValidSchema compileJsonSchemaWithNamedReferences(std::istream &is,
                                                            const std::map<Name, ValidSchema> &namedReferences);
 
 } // namespace avro

lang/c++/include/avro/Config.hh

@@ -21,17 +21,29 @@
 
 // Windows DLL support
 
-#ifdef _WIN32
+#ifdef _MSC_VER
 #pragma warning(disable : 4275 4251)
+#endif // _MSC_VER
+
+#if defined _WIN32 || defined __CYGWIN__
+#define AVRO_DLL_EXPORT __declspec(dllexport)
+#define AVRO_DLL_IMPORT __declspec(dllimport)
+#define AVRO_DLL_HIDDEN
+#else
+#define AVRO_DLL_EXPORT [[gnu::visibility("default")]]
+#define AVRO_DLL_IMPORT [[gnu::visibility("default")]]
+#define AVRO_DLL_HIDDEN [[gnu::visibility("hidden")]]
+#endif // _WIN32 || __CYGWIN__
 
-#if defined(AVRO_DYN_LINK)
+#ifdef AVRO_DYN_LINK
 #ifdef AVRO_SOURCE
-#define AVRO_DECL __declspec(dllexport)
+#define AVRO_DECL AVRO_DLL_EXPORT
 #else
-#define AVRO_DECL __declspec(dllimport)
+#define AVRO_DECL AVRO_DLL_IMPORT
 #endif // AVRO_SOURCE
 #endif // AVRO_DYN_LINK
 
+#ifdef _WIN32
 #include <intsafe.h>
 using ssize_t = SSIZE_T;
 #endif // _WIN32