build: build libuv.a and libuv.so in different directories

[bnoordhuis]

See e.g. #632.

Running make all && make test fails to build the test suite.

make all first builds libuv.a, make test then tries to build libuv.so but dies with relocation errors.

Possible solutions:

1. Always build with -fPIC. Bad but easy.
2. Build libuv.a and libuv.so in different directories. Good but hard(er).

[bnoordhuis]

Fixed in 4b115f8.