Suggestion: make -athread=native the default

[ksdhans]

Just tried out the C++11 threading, and it's great to have it working. However, I got a heap of "undefined reference" errors the first time I tried to compile, because I didn't use the -athread parameter. That's rather confusing, especially if you don't know about the -athread parameter.

I suggest enabling threading by default, or enabling it when C++ code uses threading (if that's feasible).

If that's not possible, then perhaps the compiler could spit out an error informing you about -athread when code that needs it is compiled?