Merge pull request #5819
d23b0a2 depends: always use static qt5 for linux (Cory Fields)
3448b13 build: fix typo in configure help (Cory Fields)
c95ac83 gitian: fix x86_64 build with static libstdc++ (Cory Fields)
0671516 build: change reduce exports/static libstdc++ options for gitian and travis (Cory Fields)
aa36730 build: remove libstdc++ backwards-compat (Cory Fields)
3ee028f build: disable reduced exports by default (Cory Fields)