Revert "C++11: OmniROM build system compatibility fix."

This reverts commit eb51e95602e282eaaa7653d5315c588571ac375a.
Fix clang errors

Change-Id: Ib482b3a4d8609302adc8020de712d1130d59ad7d
Set HAVE_UNWIND_CONTEXT_STRUCT for Clang

Change-Id: I423d650181c6736111e4a5e91ded81c471dd08e1
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>