remove hardcoded compiler optimizations and -g

Index: rts/lib/assimp/code/CMakeLists.txt
--- rts/lib/assimp/code/CMakeLists.txt.orig
+++ rts/lib/assimp/code/CMakeLists.txt
@@ -480,7 +480,7 @@ if (ASSIMP_BUILD_IFC_IMPORTER)
   if (MSVC)
     set_source_files_properties(IFCReaderGen1.cpp IFCReaderGen2.cpp PROPERTIES COMPILE_FLAGS "/bigobj")
   elseif(CMAKE_COMPILER_IS_MINGW)
-    set_source_files_properties(IFCReaderGen1.cpp IFCReaderGen2.cpp PROPERTIES COMPILE_FLAGS "-O2 -Wa,-mbig-obj")
+    set_source_files_properties(IFCReaderGen1.cpp IFCReaderGen2.cpp PROPERTIES COMPILE_FLAGS "-Wa,-mbig-obj")
   endif()
 endif (ASSIMP_BUILD_IFC_IMPORTER)
 
