CMake: add LibUV_USE_STATIC configuration variable

If LibUV_USE_STATIC is set then the static libuv library will be
preferred to the shared library. This is useful when building with the
bundled libuv but is less useful when building with a system libuv.
Makefile: use static linkage with bundled libuv

Use the new LibUV_USE_STATIC configuration flag to use static linkage
when using the bundled libuv.
Merge pull request #172 from rjw57/issue-171

use shared libuv library if not using the bundled version