[build] Fix build with default configuration for GCC <4.7

[jlsantiago0]

This fixes default build configuration for GCC<4.7. This fixes default build configuration for CentOS<7, Ubuntu<14, and Slackware<14.

This changes sets the ENABLE_CXX11 default to OFF when the compiler is GCC<4.7. c++11 was initially introduced in GCC-4.7.0. This PR allows building of OpenSRT when running cmake without -DENABLE_CXX11=OFF on those platforms.

[maxsharabayko]

Please update the --enable-c++11 option description in the "build options" document.
The default value is "ON" except for GCC 4.7 and below.