build: complete the changes from apple#423

This adds `-parse-as-library` to the example targets which use `@main`
resulting in a compile failure as `@main` is not processed unless the
target is marked as library.

Examples/CMakeLists.txt

@@ -1,11 +1,15 @@
 add_executable(math
   math/Math.swift)
+target_compile_options(math PRIVATE
+  -parse-as-library)
 target_link_libraries(math PRIVATE
   ArgumentParser
   $<$<STREQUAL:${CMAKE_SYSTEM_NAME},Linux>:m>)
 
 add_executable(repeat
   repeat/Repeat.swift)
+target_compile_options(repeat PRIVATE
+  -parse-as-library)
 target_link_libraries(repeat PRIVATE
   ArgumentParser)