diy2 build failures on Mac OS X when VTK_Group_MPI is on

The logic for defining

VTKDIY2_WARNING_PRAGMAS
VTKDIY2_WARNINGS_PUSH
VTKDIY2_WARNINGS_POP

is incomplete for GNU compilers. This causes a build failure with compiler

> g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/c++/4.2.1
Apple LLVM version 7.0.0 (clang-700.1.76)
Target: x86_64-apple-darwin15.4.0
Thread model: posix