Move definition of GLSLANG_EXPORT to visibility.h

The GLSLANG_EXPORT macro is already used (and defined) in two separate
places and will be used in a few more in the near future. To reduce
duplication it is now in its own header.

glslang/Public/ShaderLang.h

@@ -38,6 +38,7 @@
 #define _COMPILER_INTERFACE_INCLUDED_
 
 #include "../Include/ResourceLimits.h"
+#include "../Include/visibility.h"
 #include "../MachineIndependent/Versions.h"
 
 #include <cstring>
@@ -49,22 +50,6 @@
     #define C_DECL
 #endif
 
-#ifdef GLSLANG_IS_SHARED_LIBRARY
-    #ifdef _WIN32
-        #ifdef GLSLANG_EXPORTING
-            #define GLSLANG_EXPORT __declspec(dllexport)
-        #else
-            #define GLSLANG_EXPORT __declspec(dllimport)
-        #endif
-    #elif __GNUC__ >= 4
-        #define GLSLANG_EXPORT __attribute__((visibility("default")))
-    #endif
-#endif // GLSLANG_IS_SHARED_LIBRARY
-
-#ifndef GLSLANG_EXPORT
-#define GLSLANG_EXPORT
-#endif
-
 //
 // This is the platform independent interface between an OGL driver
 // and the shading language compiler/linker.