sed question

[Antony Stace <antony@example.com>]

Hi Folks

I have a config file with the LIBS defined about 250 times through out
the file.  I want to use sed to change each line which starts with 

LIBS="

and at the end of the line, before the " mark, insert

-lnetinet

, ie the line

LIBS="$ac_save_LIBS"

would become

LIBS="$ac_save_LIBS -lnetinet"

Can someone please tell me how to do this in sed. 

Cheers

Tony