[iostreams] workaround for the boost + stdcxx on MSVC 7.1

[farid_zaripov]

I have been working on the Apache C++ Standard Library project (STDCXX):
http://incubator.apache.org/stdcxx/

I've trying to compile the boost library with stdcxx.

The details can be found here:
http://mail-archives.apache.org/mod_mbox/incubator-stdcxx-dev/200704.mbox/%
3c7BDB2168BEAEF14C98F1901FD2DE64386573EB@epmsa009.minsk.epam.com%3e

Boost version: 1.33.1
STDCXX version: 4.2

I've got the error in boost\iostreams\device\mapped_file.hpp:

D:\_Libs\boost_1_33_1\boost\iostreams\device\mapped_file.hpp(67) : error C2065: '__rw_openmode' : undeclared identifier
D:\_Libs\boost_1_33_1\boost\iostreams\device\mapped_file.hpp(67) : error C2064: term does not evaluate to a function taking 0 arguments
D:\_Libs\boost_1_33_1\boost\iostreams\device\mapped_file.hpp(72) : error C2064: term does not evaluate to a function taking 0 arguments

    CALL "C:\Program Files\Microsoft Visual Studio .NET 2003\VC7\bin\VCVARS32.BAT" >nul 
    "C:\Program Files\Microsoft Visual Studio .NET 2003\VC7\bin\cl"  /Zm800 -nologo /EHsc -c  -D_RWSTDDEBUG  /Z7 /Od /Ob0 /EHsc /GR /MTd /Op  /wd4675 /Zc:forScope  -I"bin\boost\libs\iostreams\build" -I"D:\_Libs\boost_1_33_1"  -I"D:\_Projects\stdcxx_working\include" -I"D:\_Projects\stdcxx_working\include\ansi" -I"D:\_Projects\stdcxx_working\build\msvc-7.1\15s\include"  -I"D:\_Libs\boost_1_33_1"   -Fo"bin\boost\libs\iostreams\build\libboost_iostreams.lib\vc-7_1-stdcxx\debug\runtime-link-static\threading-multi\mapped_file.obj"  -Tp"D:\_Libs\boost_1_33_1\libs\iostreams\build\..\src\mapped_file.cpp" 


This error issued because of the following bug of the MSVC 7.1:
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=267488

The proposed patch is attached.