Fix #554 Use CMAKE_BUILD_TYPE for splash2txt

 - use CMAKE_BUILD_TYPE instead of S2T_RELEASE cmake option
 - fix a compile error with gcc 4.9 (uint32_t undefined)

Fix for #554 
oh. since `splash2txt` does not use the usual cuda-cmake weirdness, you should be able to suppress the header warnings of third party libs via
```cmake
include_directories(SYSTEM ${Boost_INCLUDE_DIRS})
```
in line [119](https://github.com/f-schmitt-zih/picongpu/blob/topic-cmakebuildtype/src/tools/splash2txt/CMakeLists.txt#L119) - can you add and confirm that?