build: add support for Swift toolchains not provided by Apple

the xcode-select tool only properly works with Apple provided toolchains
but not with third party ones from swift.org. in the latter case the
swift compiler executable is found in the proper path but the swift libs
from the xcode or command line tools will be picked. this leads to a
not wanted discrepancy of the swift compiler and libs and possible
errors.

instead of relying on the xcode-select tool search for the libs relative
to the swift executable. that relative path seems to be the same for all
toolchains. if for any reasons a swift executable is not found in the
relative path, fall back to the old xcode-select method.

furthermore, both static and dynamic libs will be searched for but only
the former will be used for now. this is a preparation for the upcoming
swift 5 migration.

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