Check for loop variable initialisation support in configure

[MeisterEule]

Oftentimes, the compilation fails because some compilers do not support loop variable initialisation (for (int i = 0...)) per default. Rather, they require an explicit --std=gnu99 flag, or similar. We could check during configuration if this occurs and either supply the corresponding flag autmatically (which might be a bit risky because the names of the flag can differ), or abort the configuration and require the user to add the flag.

[SpinTensor]

The flags -std=gnu11, -std=c11, -std=c99 are now check in configure until the compiler accepts one. If none is accepted configure aborts with an error message.