GNUtoMS: Add search path for VS 2015 environment scripts

Alex Ghoshrequested to merge
avacar/cmake:Windows-GNU_platform_fix into master

Change is necessary to bring back cmake_add_fortran_subdirectory functionality to Visual Studio 2017 (and possibly 2015, which I never got to work historically). It definitely works now with this path change in the platform file.

Topic-rename: vs14-GNUtoMS

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