C++11: Upgrade auto-detection to use the formal -std=c++11

C++11: Upgrade auto-detection to use the formal -std=c++11

When the latest compilers added support for -std=c++11 they also dropped
the temporary -std=c++0x option without backward-compatible support. So
for the newest compilers we have not been testing the C++11 code.

As a result of this change Squid will no longer attempt to enable the
partial support in older compilers with -std=c++0x.

Also, update the compiler option test macro from autoconf project.