Cppcheck tool is failing from version 2.15 if "--std=" flag is given
an unknown value. For example, "--std=c++0x" is not known, but
"--std=c++11" is. Before cppcheck-2.15 the latest known standard
version was silently used.

In this patch the unknown standard versions are mapped to the known
ones.

When a C/C++ standard version is provided both in the original build
command and the --cppcheckargs file, then only the one in --cppcheckargs
should be used in the analyzer command.