Workaround to self move assign error with _GLIBCXX_DEBUG

[BrettDong]

Summary

Build "Workaround to self move assign error with _GLIBCXX_DEBUG"

Purpose of change

_GLIBCXX_DEBUG tests are failing (for example see https://travis-ci.org/github/CleverRaven/Cataclysm-DDA/jobs/758957947) because of a bug in libstdc++ (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85828) that when std::shuffle in src/item_group.cpp swaps an element to itself, the program is terminated with error

(crafting_skill_gain)=>	Error: attempt to self move assign.
(crafting_skill_gain)=>	
(crafting_skill_gain)=>	Objects involved in the operation:
(crafting_skill_gain)=>	    sequence "this" @ 0x0x12d31328 {
(crafting_skill_gain)=>	      type = __gnu_debug::_Safe_container<std::__debug::list<item_pocket, std::allocator<item_pocket> >, std::allocator<item_pocket>, __gnu_debug::_Safe_node_sequence, true>;
(crafting_skill_gain)=>	    }

Describe the solution

Add a workaround brought up by Jonathan Wakely in https://stackoverflow.com/questions/22915325/avoiding-self-assignment-in-stdshuffle that disables self move assignment check in libstdc++.

Describe alternatives you've considered

Disable _GLIBCXX_DEBUG test in Travis CI test matrix until GCC 11 is released.

Testing

Does not crash or error when compiling with CXXFLAGS=-D_GLIBCXX_DEBUG.

Additional context

The fix (https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=c2fb0a1a2e7a0fb15cf3cf876f621902ccd273f0) is in GCC master branch and will be shipped with GCC 11 release.