make AVX warnings optional

[anyeli]

The library, if compiled with DYNAMIC_ARCH=1, may print out something like this:

OpenBLAS : Your OS does not support AVX instructions. OpenBLAS is using Barcelona kernels as a fallback, which may give poorer performance.

The library user may not want to see this as,

• it is not really an error; it does not terminate the program and does not cause incorrect results
• there is nothing to do if upgrading is not an option
• it may disrupt the user if used in an interactive program
• it may break scripts that assume the program indicates errors by printing to standard error (e.g. some old Fortran programs)
• it is just annoying if the user does not care :)

Could you please make this printout optional, as a compile time option, or even as a run time option? There are 6 places in driver/others/dynamic.c where a variant of this printout occurs.

[xianyi]

@anyeli , please try the develop branch.

[anyeli]

Fix looks good, thanks!