Add warning for non-portable pragmas

[niemela]

Give a warning if example submissions in C/C++ use non-portable pragmas such as e.g.:

#pragma GCC target("bmi,bmi2,lzcnt,popcnt")
#pragma GCC target("movbe")
#pragma GCC target("aes,pclmul,rdrnd")
#pragma GCC target("avx,avx2,f16c,fma,sse3,ssse3,sse4.1,sse4.2")

It is of course true that problemtools can't check for every possible footgun one can set up, but this particular one is very common, and I think a warning would be useful.