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

external/icu/ExternalProject_icu.mk

@@ -56,7 +56,7 @@ icu_CFLAGS:=" \
 	$(if $(filter GCC,$(COM)),-fno-strict-aliasing) \
 	$(if $(filter $(true),$(gb_SYMBOL)),-g) \
 	$(if $(filter ANDROID,$(OS)),-fvisibility=hidden -fno-omit-frame-pointer)"
-icu_CXXFLAGS:=" \
+icu_CXXFLAGS:="$(CXXFLAGS_CXX11) \
 	$(if $(filter IOS,$(OS)),-DUCONFIG_NO_FILE_IO) \
 	$(if $(debug),$(gb_DEBUG_CFLAGS),$(gb_COMPILEROPTFLAGS)) \
 	$(if $(ENABLE_LTO),$(gb_LTOFLAGS)) \