In some cases, like when building the Swift toolchain build tools,
`SWIFT_STDLIB_LIBRARY_BUILD_TYPES` is not defined. This causes a
configuration failure when the `CompatibilitySpan` library is being
added.
This commit sets a default value of `STATIC` to work around this
problem.
This dynamic library contains a copy of the standard library's
exported entry points for the Span and RawSpan types. This will
allow backward deployment of code that uses those new types.
