Fix build errors after modifying CMake setup.h.in

Always set a value to ICONV_CONST so it will be defined in setup.h.
Add a check for strtoull to prevent a macro redefined warning.
Link with WebKit framework to fix macOS build with wxUSE_WEBKIT.

build/cmake/setup.cmake

@@ -221,6 +221,7 @@ if(NOT WIN32)
     if(wxUSE_LIBICONV AND NOT APPLE)
         find_package(Iconv REQUIRED)
         set(HAVE_ICONV ON)
+        set(ICONV_CONST " ")
         if(ICONV_SECOND_ARGUMENT_IS_CONST)
             set(ICONV_CONST "const")
         endif()
@@ -554,7 +555,7 @@ endforeach()
 # Check various functions
 foreach(func
     fsync
-    snprintf vsnprintf strnlen
+    snprintf vsnprintf strnlen strtoull
     setpriority
     )
     string(TOUPPER ${func} func_upper)