contrib/buildsystems: handle options starting with a slash

With the recent changes to allow building with MSVC=1, we now pass the /OPT:REF option to the compiler. This confuses the parser that wants to turn the output of a dry run into project definitions for QMake and Visual Studio: Unhandled link option @ line 213: /OPT:REF at [...] Let's just extend the code that passes through options that start with a dash, so that it passes through options that start with a slash, too. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>