Disable m3d for 5.1

- closes https://github.com/assimp/assimp/issues/4126

CMakeLists.txt

@@ -527,6 +527,9 @@ ELSE()
   ADD_DEFINITIONS ( -D'ASSIMP_BUILD_COMPILER="${ASSIMP_BUILD_COMPILER}"' )
 ENDIF()
 
+# Disabled for 5.1
+ADD_DEFINITIONS ( -DASSIMP_BUILD_NO_M3D_IMPORTER )
+
 MARK_AS_ADVANCED ( ASSIMP_BUILD_ARCHITECTURE ASSIMP_BUILD_COMPILER )
 
 SET ( ASSIMP_BUILD_NONFREE_C4D_IMPORTER OFF CACHE BOOL