CMakeLists.txt needs -std=c++1 for cmake < 3.1

[nschiraldi]

Build is failing on RHEL7 with cmake=2.8.12.2 and gcc=4.8.2

running install
running bdist_egg
running egg_info
writing src/tda.egg-info/PKG-INFO
writing dependency_links to src/tda.egg-info/dependency_links.txt
writing top-level names to src/tda.egg-info/top_level.txt
reading manifest file 'src/tda.egg-info/SOURCES.txt'
writing manifest file 'src/tda.egg-info/SOURCES.txt'
installing library code to build/bdist.linux-x86_64/egg
running install_lib
running build_ext
-- pybind11 v2.2.3
-- Configuring done
-- Generating done
-- Build files have been written to: /network/rit/lab/schiraldilab/bin/TDA/build/temp.linux-x86_64-3.6
[  8%] [  8%] Building CXX object CMakeFiles/tda.dir/algorithms/CubicalRipser_2dim/src/dense_cubical_grids.cpp.o
Building CXX object CMakeFiles/tda.dir/algorithms/CubicalRipser_2dim/src/cubicalripser_2dim.cpp.o
In file included from /usr/include/c++/4.8.2/unordered_map:35:0,
                 from /network/rit/lab/schiraldilab/bin/TDA/algorithms/CubicalRipser_2dim/src/cubicalripser_2dim.cpp:31:
/usr/include/c++/4.8.2/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support is currently experimental, and must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
#error This file requires compiler and library support for the \

To fix, added following to CMakeLists.txt:

cmake_minimum_required(VERSION 2.8)
set(CMAKE_CXX_FLAGS "-fpermissive")
if (CMAKE_VERSION VERSION_LESS "3.1")
  if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
  endif ()
else ()
  set (CMAKE_CXX_STANDARD 11)
endif ()