build: add Swift dynamic linking support

this is in preparation for the upcoming swift 5 transition, where static
linking was replaced by dynamic linking the swift libraries as the
preferred way, by Apple. furthermore Apple removed the static swift libs
from their dev Tools starting with xcode 10.2/swift 5.

because of ABI incompatibility dynamic linking for swift versions prior
to 5 doesn't use the system lib path for the dynamic swift libs.

for now static linking is still the default, but that will be changed
when swift 5 support is added and swift 3 support is dropped.

Fixes #6232

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