Fix: Turn off unused function warning if compiling with clang on macOS

Xcode 10 started treating the swap template specialization in the cpp as
an unused function. Because warnings are treated as errors with -Werror,
this caused the build to fail.

CMakeLists.txt

@@ -44,6 +44,11 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     set(WARNINGS "-Werror" "-Wall")
 elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     set(WARNINGS "-Werror" "-Wall")
+    if (APPLE)
+      # Xcode 10 started treating the swap template specialization in the cpp as
+      # an unused function and causing an error, so turn off the warning
+      list(APPEND WARNINGS "-Wno-unused-function")
+    endif()
 elseif(MSVC)
     set(WARNINGS "/WX" "/W4")
 endif()