cmake: update FindTinyXML.cmake
cmake: require >=tinyxml-2.6

fixes #393
cmake: mac os X requires cmake 2.8.10

prior versions are shipped without SSL support (used by ExternalProject)

fixes #418
Merge pull request #419 from karolherbst/cmake

cmake: mac os X requires cmake 2.8.10
Merge pull request #412 from karolherbst/bug393

fix bug393