[msvc] Honor %INCLUDE%. It should know essential search paths with vcvarsall.bat.

FYI,

On VS10, %INCLUDE% contains;
(VS10)\VC\INCLUDE
(VS10)\VC\ATLMFC\INCLUDE
(SDK70A)\include

On VS11,
(VS11)\VC\INCLUDE
(VS11)\VC\ATLMFC\INCLUDE
(SDK80)\include\shared
(SDK80)\include\um
(SDK80)\include\winrt

FIXME: It may be enabled also on mingw.
llvm-svn: 152589