Variable length array of std::strings (not legal in C++) changed to std::vector<std::string>

Variable-length arrays (VLAs) are not permitted in standard C++.

Some compiler support them as extension:
e.g. Clang supports VLAs in a limited way (only with POD datatypes).

Here is a great summary of the issue and how to solve it:
http://clang.llvm.org/compatibility.html#vla

This pull request does exactly this: make standard conforming compilers happy.

(Specifically clang-3.4 is now able to compile the project)