Fix Xdmf3 CMake file to export symbols in DLL.

The CMake was defining/using the variable BUILD_SHARED_LIBS
while the XdmfConfig.hpp.in file use BUILD_SHARED.

Change-Id: Ibf8e3eb004b8506207197aee8892c3b34e4bc166
Merge topic 'fix_xdmf3_dll_export' into master

c9091891 Fix Xdmf3 CMake file to export symbols in DLL.