Update utility.cc

Fixed error: expected unqualified-id before ‘__extension__’
 static Complex I(0.0,1.0), C1(1.0,0.0),C0(0.0,0.0);
                ^
utility.cc:453:16: error: expected ‘)’ before ‘__extension__’
with PLATFORM=lapack
Merge pull request #56 from jalfonsi/patch-3

Update utility.cc