Fix #pragma warning push/pop mismatch in multi_span (Issue #431) 
Fix #pragma warning push/pop mismatch in multi_span (Issue #431) 

Rearranged preprocessor code at end of file multi_span
martinmoene/GSL,garyfurnish/GSL,bashrc-real/GSL,kernhanda/GSL,neilmacintosh/GSL,Josaphat/GSL