Merge pull request #4042
05c20a5 build: add symbol for upcoming gcc 4.9's libstdc++ (Cory Fields)
49a3352 gitian-linux: --enable-glibc-back-compat (Warren Togami)
d5aab70 build: add an option for enabling glibc back-compat (Cory Fields)
ffc6b67 build: add glibc/libstdc++ back-compat stubs (Cory Fields)