CI: Add an option to make CI fail by compiler warnings.

If the `fail_by_warn` is `true` in the case on CI, the case fails by the
compiler warnings by the `rake compile`.

.github/workflows/test.yml

@@ -19,6 +19,7 @@ jobs:
         # ubuntu-22.04 uses OpenSSL 3.0, ubuntu-20.04 uses OpenSSL 1.1.1
         os: [ ubuntu-22.04, ubuntu-20.04, macos-latest, windows-latest ]
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
+        fail_by_warn: [ true ]
         exclude:
           # uses non-standard MSYS2 OpenSSL 3 package
           - { os: windows-latest, ruby: head }
@@ -47,6 +48,10 @@ jobs:
         run:  echo "MAKEFLAGS=V=1" >> $GITHUB_ENV
         if: runner.os == 'Linux' || runner.os == 'macOS'
 
+      - name: set flags to fail by the compiler warnings.
+        run:  echo "RUBY_OPENSSL_EXTCFLAGS=-Werror" >> $GITHUB_ENV
+        if: matrix.fail_by_warn
+
       - name: compile
         run:  rake compile -- --enable-debug
 
@@ -79,6 +84,7 @@ jobs:
           - libressl-3.6.3
           - libressl-3.7.3
           - libressl-3.8.0 # Development release
+        fail_by_warn: [ true ]
         fips-enabled: [ false ]
         include:
           - { os: ubuntu-latest, ruby: "3.0", openssl: openssl-3.0.9, fips-enabled: true, append-configure: 'enable-fips', name-extra: 'fips' }
@@ -142,6 +148,10 @@ jobs:
         run:  echo "MAKEFLAGS=V=1" >> $GITHUB_ENV
         if: runner.os == 'Linux' || runner.os == 'macOS'
 
+      - name: set flags to fail by the compiler warnings.
+        run:  echo "RUBY_OPENSSL_EXTCFLAGS=-Werror" >> $GITHUB_ENV
+        if: matrix.fail_by_warn
+
       - name: compile
         run:  rake compile -- --enable-debug --with-openssl-dir=$HOME/.openssl/${{ matrix.openssl }}