[CMAKE] Refactored FindXDR.cmake module

[philthiel]

Configuration problems on macOS using -std=c++11 concerning finding XDR should be resolved now. This should fix #639.

[tkemmer]

The FindXDR module seems to work now! However, FindBoost stopped working with your PR on my Mac (Sierra, CMake 3.8, Contrib) for some reason.

Master:

-- Performing Test BALL_XDRREC_CREATE_VOID_VOID_INT - Success
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - found
-- Found Threads: TRUE  
-- Boost version: 1.60.0

PR:

-- Performing Test BALL_XDRREC_CREATE_VOID_VOID_INT - Success
-- Looking for pthread.h
-- Looking for pthread.h - not found
CMake Error at /Applications/CLion.app/Contents/bin/cmake/share/cmake-3.8/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
  Could NOT find Threads (missing: Threads_FOUND)
Call Stack (most recent call first):
  /Applications/CLion.app/Contents/bin/cmake/share/cmake-3.8/Modules/FindPackageHandleStandardArgs.cmake:377 (_FPHSA_FAILURE_MESSAGE)
  /Applications/CLion.app/Contents/bin/cmake/share/cmake-3.8/Modules/FindThreads.cmake:212 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  /Applications/CLion.app/Contents/bin/cmake/share/cmake-3.8/Modules/CMakeFindDependencyMacro.cmake:55 (find_package)
  /Applications/CLion.app/Contents/bin/cmake/share/cmake-3.8/Modules/FindBoost.cmake:1478 (find_dependency)
  cmake/BALLConfigBoost.cmake:38 (FIND_PACKAGE)
  CMakeLists.txt:133 (INCLUDE)

This problem does not seem to occur with our Sierra and El Capitan build workers, though.

[philthiel]

CI runs are ok: on macOS it is only the Timer_test that failed. Not related to XDR.

The boost thing is weird, I will look into that.