fix(libassuan)

closes #6447

projects/gnupg.org/libassuan/package.yml

@@ -20,6 +20,12 @@ build:
     make install
   dependencies:
     gnupg.org/libgpg-error: 1
+  env:
+    darwin:
+      # otherwise it tries to link both libc++ and libstdc++, which gives
+      # duplicate symbol errors.
+      CFLAGS: $CFLAGS -std=gnu89
+      CXXFLAGS: '$CXXFLAGS -stdlib=libstdc++ -std=c++03'
 
 test:
   script: test "$(libassuan-config --version)" = "{{version}}"