C++ fixes for old gcc (SLE_11, RHEL-6)

was broken since 2d2e16a8
Merge pull request #63 from rudimeier/fix-old-cpp

C++ fixes for old gcc (SLE_11, RHEL-6)
Do not use at CMakeLists.txt [ as cmake 3 dont like it. Use test.
Merge branch 'master' of github.com:davidmoreno/onion
Another cmake 3 fix regarding command [ and test.