Use lib instead of ar on Windows

[everythingfunctional]

If ar isn't available, use lib to create static library files. Only do this check on Windows (or Unknown) systems. It's almost certainly safe to assume ar exists on all other systems.

Fix #384

[LKedward]

Thanks @everythingfunctional. I think this looks good for now to get improved support on Windows.

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In the future, this functionality may be better placed in fpm_compiler.f90 in order to invoke compiler-specific library tools (e.g. gcc-ar, xiar, xilib) needed for LTO.

[everythingfunctional]

Having addressed the one comment, and with another approval, I'm going to go ahead and merge this. Hopefully it's not too difficult for #449 and #451 to incorporate this.