The sed implementation in GNU varies somewhat from the BSD sed implementation. Here is a patch that provides some detection for the two in configure.ac and also uses macros in the docs/Makefile.am so that we don't assume that sed is named "sed". Particularly, BSD and GNU differ on how -i works.
sed -i 's/regex/replace/' filename
sed -i '' 's/regex/replace/' filename
In BSD, the argument to -i is mandatory, thus must be explicitly specified with an empty string to indicate "in-place editing". For GNU, it is optional, and the empty-string modifier seems to confuse it.