build, windows: disable SEH

Turn off safe exception handlers, they're incompatible with how
openssl is compiled / linked under MSVS 2012.

Addresses the following build error:

  openssl.lib(x86cpuid.obj) : error LNK2026: module unsafe for SAFESEH
  image. [g:\jenkins\workspace\nodejs-oneoff\node.vcxproj]
  openssl.lib(x86.obj) : error LNK2026: module unsafe for SAFESEH
  image. [g:\jenkins\workspace\nodejs-oneoff\node.vcxproj]
  # etc. etc.
  g:\jenkins\workspace\nodejs-oneoff\Release\node.exe : fatal error
  LNK1281: Unable to generate SAFESEH image.
  [g:\jenkins\workspace\nodejs-oneoff\node.vcxproj]

Fixes #4242.