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/Include/visibility.h

@@ -0,0 +1,51 @@
+//
+// Copyright (C) 2023 LunarG, Inc.
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions
+// are met:
+//
+//    Redistributions of source code must retain the above copyright
+//    notice, this list of conditions and the following disclaimer.
+//
+//    Redistributions in binary form must reproduce the above
+//    copyright notice, this list of conditions and the following
+//    disclaimer in the documentation and/or other materials provided
+//    with the distribution.
+//
+//    Neither the name of 3Dlabs Inc. Ltd. nor the names of its
+//    contributors may be used to endorse or promote products derived
+//    from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+// FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+// COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+// INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+// BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+// ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+// POSSIBILITY OF SUCH DAMAGE.
+//
+#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
+
+