On Windows, we set INSTALLTOP to default as follows:

  VC-WIN32:

    PREFIX:     %ProgramFiles(x86)%\OpenSSL
    OPENSSLDIR: %CommonProgramFiles(x86)%\SSL

  VC-WIN64*:

    PREFIX:     %ProgramW6432%\OpenSSL
    OPENSSLDIR: %CommonProgramW6432%\SSL

Should those environment variables be missing, the following is used
as fallback:

    PREFIX:     %ProgramFiles%\OpenSSL
    OPENSSLDIR: %CommonProgramFiles%\SSL
Reviewed-by: NAndy Polyakov <appro@openssl.org>