Fixed freeglut.pdb install for builds with MSVC compiler and NMake.

The CMake variable "MSVC" indicates that the Visual Studio compiler is
used but  when using that compiler and the "NMake Makefiles JOM" build
generator the *.pdb is wasn't output in a "Debug" directory.

(cherry picked from commit 4e6bceed00b2c64a3cfc0bcbf84350f656a4a7c3)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1834 7f0cb862-5218-0410-a997-914c9d46530a