Starting with 0.5.11 calibre depends on pdftk. However, *none* of the versions of pdftk in Portage will build with gcc 4.4 -- bug 269312 is for 1.41-r1 but I can confirm 1.41 and 1.12 won't build either. Also bug 264510 is for build problems with earlier gcc versions.
When 0.5.12 was added, 0.5.10 (the version I run) was removed. So currently anyone using gcc 4.4.0 will be forced to downgrade to 0.4.77.
0.5.10 should be added back to the tree so a recent version is available. Perhaps a USE flag controlling the use of pdftk as a dependency should be added but that would clearly be more work. In either case, the current situation with pdftk failing to build with various gcc versions makes it unsuited as a hard dependency.
And less than an hour later, a patch that fixes building pdtfk with gcc 4.4 is posted on bug 269312. Talk about timing...
*** This bug has been marked as a duplicate of bug 269312 ***