Sets -std=c++11 in the base compiler flags for all DR code.
Refactors configure_DynamoRIO_common(), which used flags set for either C or C++ depending on the whole target, to be merged and invoked later with _DR_set_compile_flags(), where we can split the flags by source file rather than target and have two sets of flags.
Fixes misc warnings with C++11 in various files.
Fixes #2020 (closed)