Windows: unset MAKEFLAGS before calling nmake
Windows: unset MAKEFLAGS before calling nmake

Bug before this patch:
$ tools\make third_party WINDOWS_PATH_TO_PYTHON=c:\python27-64
[...]
cd dependencies\sources\zlib-1.2.11 && nmake -f win32\Makefile.msc zlib.lib
NMAKE : fatal error U1065: invalid option 'W'

google/or-tools,google/or-tools,or-tools/or-tools,google/or-tools,google/or-tools,or-tools/or-tools,or-tools/or-tools,or-tools/or-tools,google/or-tools,google/or-tools,or-tools/or-tools,or-tools/or-tools