Added quotes to CMake configuration to avoid some parsing issues when ${VERSION_STRING} and ${VERSION_MAJOR} in set_target_properties.

When building on a clean system in most cases CMake would fail with;
```
$ cmake --version
cmake version 2.8.12.2
....
CMake Error at src/core/CMakeLists.txt:124 (set_target_properties):
  set_target_properties called with incorrect number of arguments.
```
when ${VERSION_STRING} and ${VERSION_MAJOR} in set_target_properties was put in quotes CMake worked.
e.g "${VERSION_MAJOR"