Prefer static libraries for Windows GCC.

We don't want to have to ship any extra DLLs in the NDK (specifically,
this is being added to avoid run time dependency on
libwinpthread-1.dll). We'll still have dynamic dependencies on things
like kernel32.dll, but that's expected.

Bug: http://b/24911020
Change-Id: Ie15de47d99bb9257abcab29929ec1585f765db8d