Add MSVC support

[redstrate]

I needed to compile this library on MSVC, so here's a bunch of fixes for that.

• Make PkgConfig optional
• Add .dll.a to the search path for gcrypt (not strictly necessary, but I need it for my build of gcrypt.)
• Define ssize_t on Windows
• Conditional a bunch of gcc/clang compile options

[paolostivanin]

Hello, thanks a lot for your PR! Could you please squash the commits and sign the final one?

Thanks again!

[redstrate]

Apologies that took a while, squashed + signed now :-)