build: default to enabling shared libraries
build: default to enabling shared libraries

We expect to generate the shared library right now.  In preparation to
build static libraries as well, enable the shared build by default.

parkera/swift-corelibs-foundation,parkera/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,apple/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,e78l/swift-corelibs-foundation