wrong /MD flags for debug binaries on Windows

[anton-malakhov]

The build log contains a lot of messages like:

cl /nologo /TP /EP ../../src/tbb/win64-tbb-export.def /MDd /Od /Ob0 /Zi /EHsc /GR /Zc:forScope /Zc:wchar_t /DTBB_USE_DEBUG /D__TBB_LIB_NAME=tbb_preview_debug.lib /DDO_ITT_NOTIFY /GS /volatile:iso /FS /DUSE_WINTHREAD /D_CRT_SECURE_NO_DEPRECATE /D_WIN32_WINNT=0x0502    /D__TBB_BUILD=1 /W4   -MD -GL /D__TBB_CPF_BUILD=1 /I../../src /I../../src/rml/include /I../../include > tbb.def
c:\bld\tbb_1537331949229\work\include\tbb\internal/_tbb_windef.h(48) : Warning: Recommend using /MDd if compiling with TBB_USE_DEBUG!=0
cl : Command line warning D9025 : overriding '/MDd' with '/MD'

The problem is that anaconda's compiler sets -MD flags: https://github.com/AnacondaRecipes/aggregate/blob/cabb96a386f3642943823d6626787fe131584ab0/vs2015/activate.bat#L41-L42