Fix to be compiled by clang++-3.4
Fix to be compiled by clang++-3.4

Clang++-3.4 is starting to enforce the following item of C++11 standard,
thus it's making lean not compiling:

It's illegal in C++11: §8.3.6.4 [dcl.fct.default]
"If a friend declaration specifies a default argument expression, that
declaration shall be a definition and shall be the only declaration of
the function or function template in the translation unit."

leanprover/lean,leanprover-community/lean,digama0/lean,levnach/lean,johoelzl/lean,soonhokong/lean,fgdorais/lean,avigad/lean,leodemoura/lean,johoelzl/lean,c-cube/lean,Kha/lean,avigad/lean,htzh/lean,fpvandoorn/lean2,rlewis1988/lean,soonhokong/lean-osx,sp3ctum/lean,UlrikBuchholtz/lean,dselsam/lean,soonhokong/lean,javra/lean,Kha/lean,c-cube/lean,rlewis1988/lean,leanprover/lean,rlewis1988/lean,htzh/lean,javra/lean,leanprover-community/lean,fpvandoorn/lean2,c-cube/lean,leodemoura/lean,fpvandoorn/lean,fgdorais/lean,fpvandoorn/lean,fpvandoorn/lean2,soonhokong/lean,soonhokong/travis_test,digama0/lean,avigad/lean,leanprover-community/lean,Kha/lean,sp3ctum/lean,c-cube/lean,fgdorais/lean,levnach/lean,digama0/lean,UlrikBuchholtz/lean,codyroux/lean0.1,leanprover-community/lean,fpvandoorn/lean,leanprover-community/lean,leodemoura/lean,UlrikBuchholtz/lean,sp3ctum/lean,codyroux/lean0.1,fgdorais/lean,htzh/lean,rlewis1988/lean,soonhokong/lean-windows,UlrikBuchholtz/lean,soonhokong/lean-osx,eigengrau/lean,johoelzl/lean,codyroux/lean0.1,fpvandoorn/lean2,rlewis1988/lean,Kha/lean,soonhokong/lean-windows,eigengrau/lean,rlewis1988/lean,UlrikBuchholtz/lean,soonhokong/travis_test,fgdorais/lean,Kha/lean,digama0/lean,soonhokong/lean-osx,leanprover/lean,leanprover/lean,javra/lean,leanprover/lean,eigengrau/lean,soonhokong/lean-windows,fpvandoorn/lean2,levnach/lean,avigad/lean,fgdorais/lean,dselsam/lean,soonhokong/lean,soonhokong/lean-windows,Kha/lean,codyroux/lean0.1,johoelzl/lean,soonhokong/lean,sp3ctum/lean,c-cube/lean,dselsam/lean,htzh/lean,levnach/lean,htzh/lean,dselsam/lean,digama0/lean,levnach/lean,johoelzl/lean,avigad/lean,soonhokong/travis_test,eigengrau/lean,digama0/lean,leanprover-community/lean,avigad/lean,sp3ctum/lean,fpvandoorn/lean,leodemoura/lean,javra/lean,soonhokong/lean-osx,johoelzl/lean,leanprover/lean,eigengrau/lean,fpvandoorn/lean,dselsam/lean,leodemoura/lean,soonhokong/lean-osx,leodemoura/lean,soonhokong/lean-windows,dselsam/lean,javra/lean,fpvandoorn/lean,soonhokong/travis_test