Merge bitcoin#11981: Fix gitian build after libzmq bump

a6365c5 depends: fix libzmq's needless linking against libstdc++ (Cory Fields)

Pull request description:

  Broken gitian builds were introduced with bitcoin#9254. Big thanks to @jonasschnelli for narrowing down the bisection.

  This is broken for a number of reasons, including:
  - g++ understands "-static-libstdc++ -lstdc++" to mean "link against whatever libstdc++ exists, probably shared", which in itself is buggy.
  - another stdlib (libc++ for example) may be in use

Tree-SHA512: d84968ee680f32ea799034ee516e9477fff2b1ef1b7c9a8ef1941631520ab196ecd50f5d64f3ed1c84113ead44be5e3ddf6ff2ae0277625bdeed05f1da89e017

depends/packages/zeromq.mk

@@ -29,5 +29,6 @@ define $(package)_stage_cmds
 endef
 
 define $(package)_postprocess_cmds
+  sed -i.old "s/ -lstdc++//" lib/pkgconfig/libzmq.pc && \
   rm -rf bin share
 endef