Icedove 78 requires nss > 3.53. This version is the only one building
out-of-the-box.
* gnu/packages/nss.scm (nss-3.56): New variable.
* gnu/packages/patches/nss-3.56-pkgconfig.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
Signed-off-by: Mark H Weaver <mhw@netris.org>
