glslang: Fix build when OSDependent library is missing. (OGRECave#3249)

The OSDependent glslang library is only available when it is built as
a static library, not a shared one; look for the library but do not
fail the  build when it doesn't exist.

Fixes: <OGRECave#3248>

PlugIns/GLSLang/CMakeLists.txt

@@ -19,7 +19,11 @@ elseif(DEFINED ENV{VULKAN_SDK})
     target_link_directories(Plugin_GLSLangProgramManager PUBLIC $ENV{VULKAN_SDK}/lib)
     target_link_libraries(Plugin_GLSLangProgramManager PUBLIC OgreMain shaderc_combined)
 else()
-  set(GLSLANG_LIBS glslang OSDependent SPIRV SPIRV-Tools-opt SPIRV-Tools)
+  set(GLSLANG_LIBS glslang SPIRV SPIRV-Tools-opt SPIRV-Tools)
+  find_library(GLSLANG_OSDependent OSDependent)
+  if(GLSLANG_OSDependent)
+    set(GLSLANG_LIBS ${GLSLANG_LIBS} ${GLSLANG_OSDependent})
+  endif()
   find_library(GLSLANG_HLSL HLSL)
   if(GLSLANG_HLSL)
     set(GLSLANG_LIBS ${GLSLANG_LIBS} ${GLSLANG_HLSL})