Enable multi-core compiling with Visual Studio 2005/2008 for Qt

for Visual studio 2005 mkspecs\win32-msvc2005\qmake.conf
for Visual studio 2008 mkspecs\win32-msvc2008\qmake.conf

change
QMAKE_CFLAGS = -nologo -Zm200 -Zc:wchar_t-
to
QMAKE_CFLAGS = -nologo -Zm200 -MP -Zc:wchar_t-