Update CMakeLists.txt

Changed "add_definitions" to "add_compile_options" to fix building with NMake.
Using "add_definitions" adds "/MP" to the resource compiler options which results in a fatal error when run with NMake.

CMakeLists.txt

@@ -72,7 +72,7 @@ if((CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX) AND NOT CMAKE_COMPILER_
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility=hidden -Wall" )
 elseif(MSVC)
   # enable multi-core compilation with MSVC
-  add_definitions(/MP)
+  add_compile_options(/MP)
 endif()
 
 INCLUDE (FindPkgConfig)
@@ -367,4 +367,4 @@ if(WIN32)
 		add_custom_command(TARGET UpdateAssimpLibsDebugSymbolsAndDLLs COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_BINARY_DIR}/code/Debug/assimp-${ASSIMP_MSVC_VERSION}-mtd.lib		${LIB_DIR}assimp-${ASSIMP_MSVC_VERSION}-mtd.lib VERBATIM)
 		add_custom_command(TARGET UpdateAssimpLibsDebugSymbolsAndDLLs COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_BINARY_DIR}/code/Debug/assimp-${ASSIMP_MSVC_VERSION}-mtd.pdb		${LIB_DIR}assimp-${ASSIMP_MSVC_VERSION}-mtd.pdb VERBATIM)
 	endif(MSVC12 OR MSVC14)
-endif (WIN32)
+endif (WIN32)