[Build] When building against musl, add -lfts.

Musl doesn't have fts built-in; it's in a separate library, which
we need to add if we're going to use it.

rdar://123381867

Author: al45tair

Sources/Foundation/CMakeLists.txt

@@ -164,6 +164,11 @@ target_link_libraries(Foundation
     uuid
   PUBLIC
     swiftDispatch)
+if("${CMAKE_C_COMPILER_TARGET}" MATCHES ".*-musl[^-*]$")
+  target_link_libraries(Foundation
+    PUBLIC
+      fts)
+endif()
 set_target_properties(Foundation PROPERTIES
   INSTALL_RPATH "$ORIGIN"
   BUILD_RPATH "$<TARGET_FILE_DIR:swiftDispatch>"