Fix bug when CXX_FLAGS contains an option with length less than 2 (ie -g)

On a project I am working I had on CMakeLists.txt:
```
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -g -Wall -Wextra -pedantic -Werror")
```
and since the option -g has lenght less than 2, the ```(substring tk 0 2)``` failed, causing ```cpputils-cmake``` to report a very confusing error: "Failed to create Makefile for flymake. Continue anyway." and not working. I traced the source of the problem to be the parameter ```-g``` and the proposed change fixed the issue.