message: Correctly set default visibility on non-windows compilers
  This fixes compilation of pugixml with -fvisibility=hidden. Without
  this patch, one would get lots of unresolved symbols when consuming
  pugixml as a shared library.
diff: @@ -111,7 +111,9 @@ if (BUILD_SHARED_LIBS)
  ${PUGIXML_BUILD_DEFINES}
  ${PUGIXML_PUBLIC_DEFINITIONS}
  PRIVATE
  - $<$<CXX_COMPILER_ID:MSVC>:PUGIXML_API=__declspec\(dllexport\)>)
  + $<$<CXX_COMPILER_ID:MSVC>:PUGIXML_API=__declspec\(dllexport\)>
  + $<IF:$<CXX_COMPILER_ID:MSVC>,,PUGIXML_API=__attribute__\(\(visibility\("default"\)\)\)>
  + )
  target_compile_options(pugixml-shared
  PRIVATE
  ${msvc-rt-mtd-shared}
  