ci: install libtool for macosx (#1012)

* ci: install libtool for macosx

* ci: add gap stable-4.13 to ci.yml

* ci: exclude macos / GAP 4.12

.github/workflows/ci.yml

@@ -26,14 +26,21 @@ jobs:
         gap-branch:
           - master
           - stable-4.12
+          - stable-4.13
         ABI:
           - 64
+        exclude:
+          # We exclude this combination for the reasons discussed at:
+          # https://github.com/semigroups/Semigroups/pull/1015
+          # https://github.com/gap-system/gap/issues/5640
+          - os: macos
+            gap-branch: stable-4.12
 
     steps:
       - uses: actions/checkout@v4
       - name: "Install dependencies (macOS only!)"
         if: ${{ runner.os == 'macOS' }}
-        run: brew install automake
+        run: brew install automake libtool
       # Setup ccache, to speed up repeated compilation of the same binaries
       # (i.e., GAP and the packages)
       - name: "Setup ccache"