The two alternatives ("const T& t" or "T& t") both don't work because the former fails for "auto_ptr<*>" while the latter fails for "int". Forcing a copy works for both. It's less efficient, but for small (cache line or less) sized object, it's a non-issue.
Summary: In the change a3e3b2a, PKG_CHECK_MODULES is updated. However, libe.pc also need to update.