A missing header (acconf.h) issue was introduced in 34575a9786
In 419386aadd, some new source files have been added but not added to the file list in Makefile
It's modestly faster, significantly more type-safe, and doesn't assert
when there's too few arguments, which causes problems for plural forms.
Closes#1733.
ptr_vector hasn't been updated for C++11, so despite being specifically
designed to store pointers to objects it's less safe and not really any
easier to use than a regular vector of unique_ptrs