Visual Studio 2017 support

[smanders]

find script and xpGetCompilerPrefix need new conditional (for prefix)

• https://github.com/smanders/externpro/blob/18.04.1/modules/Findscript.cmake.in#L9-L30
• https://github.com/smanders/externpro/blob/18.04.1/modules/xpfunmac.cmake#L651-L672

instructions to build externpro projects need similar changes, for example:

• boost https://github.com/smanders/externpro/blob/18.04.1/projects/boost.cmake#L218-L235

externpro projects (patches) need similar changes, for example:

• wx https://github.com/smanders/wxcmake/blob/wx31/wx.cmake#L29-L52
• wxx https://github.com/smanders/wxx/blob/xpro/CMakeLists.txt#L72-L95

[smanders]

NOTE: a future version of cmake (later than current v3.11.1) will introduce a new MSVC_TOOLSET_VERSION variable that should be useful in the locations above...
Kitware/CMake@304f493

[smanders]

in looking for the next thing after MSVC14 in cmake-variables it appears that

• use of MSVC14 is discouraged
  • https://cmake.org/cmake/help/v3.9/variable/MSVC14.html
• we should use MSVC_VERSION instead
  • https://cmake.org/cmake/help/v3.9/variable/MSVC_VERSION.html

[smanders]

also, when I run cmake --help on Windows with cmake 3.9.6 to look at the list of generators I noticed

Visual Studio 8 2005 [arch] = Deprecated.

so I plan to drop support for this and anything older...

[smanders]

completed with commit to dev branch referenced above

issue for projects that don't build with Visual Studio 2017 opened #189