Fix issue #1175: allow WIN32_INSERT_DEF=0 to turn off --output-def in mingw link.
Fix issue #1175: allow WIN32_INSERT_DEF=0 to turn off --output-def in mingw link.

timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,timj/scons,timj/scons,timj/scons,timj/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,timj/scons