replace output.h macro magic with template magic

[Olly Betts]

I replaced common/output.h and common/output-internal.h with a single template in [feb4ce4011f437e81b3d5a05f239d6735491adfc/git].

This needs C++11, but GCC 4.7 doesn't seem to handle this case, and despite spending a while trying a lot of different approaches, I can't get this to work properly with GCC 4.7.

Aside from this issue, GCC 4.7 has seemed a sensible minimum requirement for 1.4.x, as it has decent C++11 support in general, and so isn't too onerous to code for. It's the newest GCC version in Debian wheezy (now in LTS support), which is a motivation for supporting it.

This doesn't seem enough of an issue to bump the GCC requirment over, so I'm going to revert this commit, and am opening this ticket so we remember to reapply it to master once we create a 1.4 branch.

Marking as 1.4.x, not because we'd apply it to 1.4.x, but so I don't forget about this.

[Olly Betts]

Reapplied in [24f51ee31c2971e54450a63542aadc87e0fdbcbd], which will debut in 1.5.0, when we get to that. I'll update the minimum GCC version requirements for master to GCC 4.8.