Build with hidden symbols by default

Turn on options to tell CMake to use -fvisibility=hidden by default (on
applicable platforms). This has some optimization benefits, and more
closely mirrors the Windows behavior of only exporting symbols marked
for export, which can help catch errors doing so on non-Windows
platforms.

This requires CMake 2.8.12 to be effective. (Otherwise, the logic will
simply have no effect.)

CMakeLists.txt

@@ -64,6 +64,9 @@ set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DDEBUG")
 option(BUILD_SHARED_LIBS "build as shared library" ON)
 option(BUILD_STATIC_LIBS "build as static library" OFF)
 
+set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
+
 if(BUILD_SHARED_LIBS)
 add_library(tinyxml2 SHARED tinyxml2.cpp tinyxml2.h)