Enable compilation under Cygwin and Apple

Conan

Removing demand that GKlib be a static library.

Properly escaping ``__thread=declspec(thread)``.

Removing unused ``rint`` definition.

Removing unused ``rint`` definition.

Removing definition of ``rint`` since it is not used within METIS.

Using ``add_definitions`` instead of explicit preprocessor flags.

Removed the definition of INFINITY since it is not used anywhere in METIS.

Avoiding "regex.h" when using MSVC.

Changing METIS_INSTALL to be an option so that Elemental can set it to false without it being overwritten