Fix scope on preprocessor variables for Max plugin.

plAudioCore needs to know whether we intend to build the plugin, so
the change introduced in 978edc1 was causing compilation to fail when
building the plugin.

This moves detection and definition back into the root CMakeList,
but preserves the behavior of only looking for the SDK if needed.

Sources/Tools/CMakeLists.txt

@@ -8,12 +8,7 @@ if(PLASMA_BUILD_TOOLS)
 endif()
 
 # Max Stuff goes below here...
-option(3dsm_BUILD_PLUGIN "Do we want to build the 3ds Max plugin?" OFF)
 if(3dsm_BUILD_PLUGIN)
-    find_package(MaxSDK REQUIRED)
-    add_definitions(-DBUILDING_MAXPLUGIN)
-    add_definitions(-DMAXPLUGINCODE)
-
     add_subdirectory(MaxComponent)
     add_subdirectory(MaxConvert)
     add_subdirectory(MaxExport)