Make linkcheck2 detection automatic

.github/workflows/ci.yml

@@ -57,7 +57,7 @@ jobs:
           cargo install mdbook-mermaid --version ${{ env.MDBOOK_MERMAID_VERSION }}
 
       - name: Check build
-        run: ENABLE_LINKCHECK=1 mdbook build
+        run: mdbook build
 
       - name: Deploy to gh-pages
         if: github.event_name == 'push'

README.md

@@ -57,13 +57,8 @@ The build files are found in the `book/html` directory.
 ### Link Validations
 
 We use `mdbook-linkcheck2` to validate URLs included in our documentation. Link
-checking is **not** run by default locally, though it is in CI. To enable it
-locally, set the environment variable `ENABLE_LINKCHECK=1` like in the
-following example.
-
-```console
-$ ENABLE_LINKCHECK=1 mdbook serve
-```
+checking is run locally only if the `mdbook-linkcheck2` crate is installed. Otherwise,
+a warning is printed to the user.
 
 ### Table of Contents
 

ci/linkcheck.sh

@@ -9,8 +9,8 @@ set_github_token() {
   jq '.config.output.linkcheck."http-headers"."github\\.com" = ["Authorization: Bearer $GITHUB_TOKEN"]'
 }
 
-if [ -z "$ENABLE_LINKCHECK" ] ; then
-  echo "Skipping link check."
+if ! which $LINKCHECK_BINARY &>/dev/null && [ -z "$GITHUB_EVENT_NAME" ] ; then
+  echo "WARNING: Skipping link check. Consider running 'cargo install $LINKCHECK_BINARY'."
   exit 0
 fi