BUG: Add escape sequence to quotes in CMake string

With: cmake version 2.8.12.1

I was getting the following warning:

Make Warning (dev) at /Users/blowekamp/src/Slicer/CMake/SlicerExtensionCPack.cmake:61 (include):
  Syntax Warning in cmake code at

    /Users/blowekamp/src/Slicer/Extensions/CMake/SlicerFunctionGenerateExtensionDescription.cmake:100:38

  Argument not separated from preceding token by whitespace.
Call Stack (most recent call first):
  CMakeLists.txt:73 (include)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at /Users/blowekamp/src/Slicer/CMake/SlicerExtensionCPack.cmake:61 (include):
  Syntax Warning in cmake code at

    /Users/blowekamp/src/Slicer/Extensions/CMake/SlicerFunctionGenerateExtensionDescription.cmake:100:39

  Argument not separated from preceding token by whitespace.
Call Stack (most recent call first):
  CMakeLists.txt:73 (include)
This warning is for project developers.  Use -Wno-dev to suppress it.