COMP: set OTB flags before custom cmake flags

cmake_cxx_flags are set in custom build scripts (dashboard or otherwise)
This is in special cases to override some annoying warning or add
special linker options. adding OTB_REQUIRED_CXX_FLAGS after
CMAKE_CXX_FLAGS will have no effect.

Example:
set(CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS} ${OTB_REQUIRED_CXX_FLAGS})
if you try to disable wshadow warning in OTBGKSVM module, this will
not work because OTB_REQUIRED_CXX_FLAGS contains -Wshadow !
so final list will be

set(CMAKE_CXX_FLAGS -Wno-shadow -Wshadow) which will show shadow
warnings at end.