use cmake-provided MSVC_TOOLSET_VERSION

[smanders]

as noted in a previous issue https://github.com/smanders/externpro/issues/185#issuecomment-389699613 -- cmake 3.12 and greater provides a new MSVC_TOOLSET_VERSION variable (https://github.com/Kitware/CMake/commit/304f493e570a177795575b5297c4b54bf1b3d2ac) that will be useful so there won't be a need to update the mapping between MSVC_VERSION and the toolset version every time a new Visual Studio is released

• here https://github.com/smanders/externpro/blob/18.08.4/modules/Findscript.cmake.in#L10-L21
• here https://github.com/smanders/externpro/blob/18.08.4/modules/xpfunmac.cmake#L635-L646
• and here https://github.com/smanders/externpro/blob/18.08.4/projects/boost.cmake#L194-L205

[smanders]

completed with commits to dev branch referenced above