[CMake] Fix CMake 3.1 warning about new policy CMP0053.

CMake Warning (dev) at CMake/export.cmake:48 (set):
  Policy CMP0053 is not set: Simplify variable reference and escape sequence
  evaluation.  Run "cmake --help-policy CMP0053" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.

Changed old-style @variable@ replacement to ${variable}. 


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10467 ea41ed52-d2ee-0310-a9c1-e6b18d33e121