minor fixes to get it to compile on windows
- fix "INTERACE" typo in Findmuparser cmake module
- for static muparser library, need to define "MUPARSER_STATIC" (on windows)
- changed Findmuparser to do so if DUNE_USE_ONLY_STATIC_LIBS is defined
- otherwise behaviour is unchanged
- on windows mkdir() only takes one argument
- for static muparser library, need to define "MUPARSER_STATIC" (on windows)