Configure: Silence spurious warnings from Embarcadero compiler.

As described in the issues below, Embarcadero compilers emit a spurious warning
when including windows.h, if compiling with the C (not C++) language.

    http://qc.embarcadero.com/wc/qcmain.aspx?d=104595
    [BCC32 Warning] winnt.h(14966)...

    http://qc.embarcadero.com/wc/qcmain.aspx?d=127681
    Annoying warnings upon inclusion of <windows.h> in .C source file

Change-Id: Ie8014edea7e8bbdd76f4555ec2d6cd6d90490032