[CMake] CPack error : Problem checking NSIS version with command: "C:/NSIS/makensis.exe" /VERSION

[Iosif Neitzke]

Apparently NSIS 2.15 lacks a "v" in the version output string, which
causes the regular expression used in cmCPackNSISGenerator [0] to
fail.

The "v" was added back to the version string in 2.16 [1], and
presumably exists all the way through NSIS version 2.46.

[0] https://github.com/Kitware/CMake/blob/master/Source/CPack/cmCPackNSISGenerator.cxx#L435

[1] http://sourceforge.net/p/nsis/code/4550/