build: fix swift linking with upcoming xcode 11

in xcode 11 the dynamic swift libraries were moved to a separated
versioned swift folder, which can't be used for linking and only for
distribution. additional to the std dynamic swift lib folder the system
wide folder is needed for linking too.

TOOLS/dylib-unhell.py

@@ -89,8 +89,16 @@ def process_libraries(libs_dict, binary, processed = []):
 def process_swift_libraries(binary):
     command = ['xcrun', '--find', 'swift-stdlib-tool']
     swiftStdlibTool = subprocess.check_output(command, universal_newlines=True).strip()
+    # from xcode11 on the dynamic swift libs reside in a separate directory from
+    # the std one, might need versioned paths for future swift versions
+    swiftLibPath = os.path.join(swiftStdlibTool, '../../lib/swift-5.0/macosx')
+    swiftLibPath = os.path.abspath(swiftLibPath)
 
     command = [swiftStdlibTool, '--copy', '--platform', 'macosx', '--scan-executable', binary, '--destination', lib_path(binary)]
+
+    if os.path.exists(swiftLibPath):
+        command.extend(['--source-libraries', swiftLibPath])
+
     subprocess.check_output(command, universal_newlines=True)
 
     print(">> setting additional rpath for swift libraries")

waftools/detections/compiler_swift.py

@@ -44,6 +44,7 @@ def __add_dynamic_swift_library_linking_flags(ctx, swift_library):
     if StrictVersion(ctx.env.SWIFT_VERSION) >= StrictVersion("5.0"):
         ctx.env.append_value('LINKFLAGS', [
             '-Xlinker', '-rpath', '-Xlinker', '/usr/lib/swift',
+            '-L/usr/lib/swift',
         ])
 
     ctx.env.append_value('LINKFLAGS', [