ci: Ensure vendored dependencies can co-exist with system libs

Ruby's mkmf appears to list `RbConfig::CONFIG['exec_prefix']/lib` first
in the search path when building a C extension. If you have libre2
installed in that path, the linker will use that library instead of
the vendored library. This adds a test to ensure this case is covered.

Author: stanhu

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ on:
 jobs:
   compile-and-test-system-dependencies:
     needs: ["build-cruby-gem"]
-    name: System Dependencies - Ruby ${{ matrix.ruby }} - libre2 ABI version ${{ matrix.libre2.soname }}
+    name: System Dependencies - ${{ matrix.sys }} vendored libs - Ruby ${{ matrix.ruby }} - libre2 ABI version ${{ matrix.libre2.soname }}
     runs-on: ubuntu-20.04
     strategy:
       matrix:
@@ -41,6 +41,7 @@ jobs:
             soname: 10
           - version: "20230701"
             soname: 11
+         sys: ["enable", "disable"]
     steps:
       - uses: actions/checkout@v3
       - name: Remove any existing libre2 installation
@@ -62,7 +63,9 @@ jobs:
         with:
           name: cruby-gem
           path: gems
-      - run: ./scripts/test-gem-install gems --enable-system-libraries
+      - name: Add a symlink for libre2
+        run: ln -s /usr/lib/libre2.so `ruby -e "puts RbConfig::CONFIG['exec_prefix']"`/lib/libre2.so
+      - run: ./scripts/test-gem-install gems --${{matrix.sys}}-system-libraries
 
   compile-and-test-vendored-dependencies:
     name: Vendored Dependencies - Ruby ${{ matrix.ruby }} - ${{ matrix.runs-on }}