FindCUDA: Fix regression under Visual Studio generators

Since commit v3.5.0-rc1~47^2 (FindCUDA: Support special characters in
path, 2016-01-15) our add_custom_command calls use VERBATIM so that
CMake will automatically quote special characters correctly.  However,
this breaks the special `$(VCInstallDir)` placeholder used with Visual
Studio generators.  Since we do not support preservation of such
placeholders with VERBATIM (see issue #15001) we must fall back to not
using VERBATIM when the placeholder is used.

A better fix would be to stop using `$(VCInstallDir)` and use the value
of `CMAKE_${CUDA_C_OR_CXX}_COMPILER` instead, but that will require
additional semantic and documentation changes.  For now simply fix the
regression with the above approach.

Reported-by: Stephen Sorley <Stephen.Sorley@jhuapl.edu>
1 file changed