Symbol visiblity

The commits defines a SPDLOG_API to mark classes that should
be exported when built as part of a shared library.
Only the registry_t class is marked with SPDLOG_API for now.

A new entry in tweakme.h has been added,
SPDLOG_ENABLE_DLL_IMPORT_EXPORT which default to undefined.
If defined, the import / export mechanism will be enabled and
if SPDLOG_EXPORTS is defined, the SPDLOG_API will export the
symbols, otherwise it will import them.

The default behavior is unchanged and SPDLOG_API will
expand to nothing.

See #693

include/spdlog/details/registry.h

@@ -25,7 +25,7 @@
 namespace spdlog {
 namespace details {
 template<class Mutex>
-class registry_t
+class SPDLOG_API registry_t
 {
 public:
     registry_t<Mutex>(const registry_t<Mutex> &) = delete;

include/spdlog/tweakme.h

@@ -141,3 +141,28 @@
 //
 // #define SPDLOG_LEVEL_NAMES { "MY TRACE", "MY DEBUG", "MY INFO", "MY WARNING", "MY ERROR", "MY CRITICAL", "OFF" }
 ///////////////////////////////////////////////////////////////////////////////
+
+///////////////////////////////////////////////////////////////////////////////
+// Uncomment to enable dll symbols export/import.
+// You also need to define SPDLOG_EXPORTS when compiling shared library.
+// 
+// #define SPDLOG_ENABLE_DLL_IMPORT_EXPORT
+///////////////////////////////////////////////////////////////////////////////
+
+///////////////////////////////////////////////////////////////////////////////
+// This defines the SPDLOG_API macro that allows a class marked with it to be
+// exported by a dll.
+// Users should define SPDLOG_EXPORTS=1 when compiling a shared library that
+// uses spdlog.
+#ifdef SPDLOG_ENABLE_DLL_IMPORT_EXPORT
+#ifdef  SPDLOG_EXPORTS
+/*Enabled as "export" while compiling the dll project*/
+#define SPDLOG_API __declspec(dllexport)
+#else
+/*Enabled as "import" in the Client side for using already created dll file*/
+#define SPDLOG_API __declspec(dllimport)
+#endif
+#else
+#define SPDLOG_API
+#endif
+///////////////////////////////////////////////////////////////////////////////