Use replace to change -O3 to -O2 so other arguments are kept intact.

CMake/ExternalAssimp.cmake

@@ -13,7 +13,9 @@ SET(assimp_CMAKE_ARGS
 )
 
 IF(APPLE)
-LIST(APPEND assimp_CMAKE_ARGS -DCMAKE_CXX_FLAGS_RELEASE="-O2")
+    # Work around a compiler crash building assimp on Mac
+    STRING(REPLACE "-O3" "-O2" assimp_CXX_FLAGS ${CMAKE_CXX_FLAGS_RELEASE})
+    LIST(APPEND assimp_CMAKE_ARGS -DCMAKE_CXX_FLAGS_RELEASE=${assimp_CXX_FLAGS})
 ENDIF(APPLE)
 
 ExternalProject_Add(assimp