Fix implicit string cast to boolean warning (treated as error)

CMakeLists.txt

@@ -226,6 +226,7 @@ else()
     if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
        CLANG_VERSION_STRING VERSION_GREATER_EQUAL 5)
         set_cpp_flag("-Werror=unused-lambda-capture")
+        set_cpp_flag("-Werror=string-conversion")
     endif()
 
     # Remove xor, and, or and friends from the list of keywords, they are used

src/Reactor/SubzeroReactor.cpp

@@ -3374,7 +3374,7 @@ namespace rr
 	#ifdef UNIMPLEMENTED
 	#undef UNIMPLEMENTED
 	#endif
-	#define UNIMPLEMENTED(msg) assert(!msg)
+	#define UNIMPLEMENTED(msg) assert(((void)(msg), false))
 
 	RValue<Float4> Sin(RValue<Float4> x) { UNIMPLEMENTED("Subzero Sin()"); return Float4(0); }
 	RValue<Float4> Cos(RValue<Float4> x) { UNIMPLEMENTED("Subzero Cos()"); return Float4(0); }