Removed another VC++ warning on x64.

Thanks to Johannes Göttker-Schnetmann.

git-svn-id: svn://svn.code.sf.net/p/quantlib/code/trunk/QuantLib@18501 8618b1d8-e22c-0410-b026-96a5dba3e089
Prevented macro redefinitions in VC++.

This should be a bit more robust against inclusions of standard
headers before or after QuantLib headers.

git-svn-id: svn://svn.code.sf.net/p/quantlib/code/trunk/QuantLib@18502 8618b1d8-e22c-0410-b026-96a5dba3e089