ApproxAFunc2Var: numerical bug triggered by compiler optimization

Here Result and Constraints UpdateInX method expect exactly the same value.
Because of the compiler optimization the evaluation of
myFirstParInU+iint*deltu may be different between Result and Constraints
update call.
Merge pull request #486 from jeromerobert/ApproxAFunc2Var_gcc_optim_bug

ApproxAFunc2Var: numerical bug triggered by compiler optimization