Tunnel CXXFLAGS_CXX11 into ICU

for our -D__float128=void hack for Clang against libstdc++; it is OK that that
explicitly enables C++11 for ICU, as ICU's configure.ac would set -std=c++0x if
no -std= is passed in.

Change-Id: I0e5044773c3d6923e3b100e19b5b54ab9edf7a1b