Improve the handling of gmock/gtest

Also started work on FindQuickBook.cmake, it needs more helpful messages
Cleanup: add THIRDPARTY_ROOT var and use a more portable way to enable c++11
Merge branch 'gc/cmakeImprovements' into intrusive_tree
Fix to be able to build performance tests with clang
Merge branch 'intrusive_tree_clangFixes' into intrusive_tree