BUG: Set RTK_BUILD_SHARED_LIBS when building inside ITK
BUG: Set RTK_BUILD_SHARED_LIBS when building inside ITK

Looking at line l95 of ITK/CMake/ITKSetStandardCompilerFlags.cmake,
BUILD_SHARED_LIBS can be set to OFF by ITK. In this case we should relay
on the ITK_LIBRARY_BUILD_TYPE value.

SimonRit/RTK,SimonRit/RTK,SimonRit/RTK,SimonRit/RTK