fish: update for new documentation build process.

Part of an updated documentation build process, the lexicon_filter script's `sed` shebang is set during make using `which`. In Homebrew's 'superenv' this will be incorrect as the superenv stub isn't
shebangable, so it needs to be rewritten.

I chose to use `inreplace` rather than a patch as it has a unique signature in Makefile.in and require less maintenance in future, won't change anything in the current 2.1.0 distribution and the change is exclusively for Homebrew.

`brew install fish --HEAD` now builds the new documentation successfully as long as the Doxygen dependency is met.
That is true, though the situation here is a little more complex and I don't think adding a parameter to configure would be appropriate.

The Makefile is setting the shebang in an external sed script, which is then populated with additional content (fish builtins, shell commands, colours etc) harvested from the source code. This script is then used by Doxygen during make as an input filter (via popen, hence the shebang requirement) to format and colourise aspects the documentation. It's very much part of a closed loop process inside the Makefile, and once make completes has no further impact on fish itself.

As such I feel that adding anything to support it to configure, aside from checking for Doxygen, would be polluting the separation between components. However, overriding a variable during make with the path to `sed` might be a good solution - would that be preferable?