build_osx - ensure we use bison from brew instead of the default one

.github/workflows/build_osx.yml

@@ -40,6 +40,8 @@ jobs:
           # preinstalled: xz, cmake
           brew install git bison autoconf automake libffi findutils
           echo "$(brew --prefix findutils)/libexec/gnubin" >> $GITHUB_PATH
+          # Mac is using bison 2.5 by default, but we need 3.5+ for swig.
+          echo "$(brew --prefix bison)/bin" >> $GITHUB_PATH
 
       - name: Install aws cli
         run: |

nix/build-all.py

@@ -91,6 +91,9 @@ Used environment variables:
 #                                                                             #
 #     on OS X El Capitan with homebrew:                                       #
 #          $ brew install git bison autoconf automake libffi cmake            #
+#          $ # `bison` shipped with Mac is too old for swig build,            #
+#          $ # so we use `brew`.                                              #
+#          $ export PATH=$(brew --prefix bison)/bin:$PATH                     #
 #                                                                             #
 #     on RHEL-related distros:                                                #
 #          $ dnf install git gcc gcc-c++ autoconf bison make cmake            #