fix c++11 issue reported by clang with libc++ stdlib:
ostream no longer has a implicit conversion operator to void*
This behavior is detected using the boost macros
BOOST_NO_EXPLICIT_CONVERSION_OPERATORS (up to 1.50) and
BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS (from 1.51)
Successfully tested in CGAL-4.3-Ic-108 and CGAL-4.3-Ic-106
Approved by the interim release manager