build: Use -std=gnu11
build: Use -std=gnu11

This reduces the set of warnings when building with Clang, which
currently doesn't like the typedef re-defitions from `G_DECLARE_DERIVABLE_TYPE`
etc.

Also, soon it's going to be 5 years after 2011, so I think we're OK
to update =)

Closes: #205
Approved by: cgwalters

Conan-Kudo/libhif,rpm-software-management/libdnf,Conan-Kudo/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,hughsie/libhif,Conan-Kudo/libhif,hughsie/libhif,edynox/libdnf,edynox/libdnf,rpm-software-management/libhif,rpm-software-management/libhif,rpm-software-management/libhif,edynox/libdnf,rpm-software-management/libdnf,rpm-software-management/libhif,hughsie/libhif,Conan-Kudo/libhif,hughsie/libhif,edynox/libdnf