Fix CMake build on Linux (apple#238)

- TestHelpers has a implicit dependency on Foundation through
  ArgumentParser, so it needs to link against Foundation
- The find package calls should be REQUIRED and CONFIG so they can take
  advantage of CMake configs

Author: gmittert

CMakeLists.txt

@@ -22,9 +22,9 @@ endif()
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 set(CMAKE_Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift)
 
-find_package(dispatch QUIET)
-find_package(Foundation QUIET)
-find_package(XCTest QUIET)
+find_package(dispatch CONFIG)
+find_package(Foundation CONFIG)
+find_package(XCTest CONFIG)
 
 add_subdirectory(Sources)
 if(BUILD_EXAMPLES)

Sources/ArgumentParserTestHelpers/CMakeLists.txt

@@ -5,4 +5,5 @@ set_target_properties(ArgumentParserTestHelpers PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY})
 target_link_libraries(ArgumentParserTestHelpers PUBLIC
   ArgumentParser
-  XCTest)
+  XCTest
+  Foundation)