app-office/gnucash does not build with -fno-semantic-interposition

I had to set `+fno-semantic-interposition` in `package.cflags` to get GNUCash to build using the default settings of the LTO overlay. I see GNUCash has an entry in the config files already. I am willing to make a small PR to remove this flag from the Gnucash build, if someone tells me where to stick it.