CI: Enable the verbose mode in the mkmf.rb by env MAKEFLAGS.

Enable the verbose option (variable `V`) in the `Makefile` generated by
`mkmf.rb` to print the compiling commands in the process of the `rake compile`
by setting the environment variable `MAKEFLAGS`[1] reserved by `make`. Because
it's helpful to see what compiler warnings are checked.

It seems that this implementation to enable the verbose mode only works in
Linux and macOS. Not Windows.

[1] https://www.gnu.org/software/make/manual/html_node/Variables_002fRecursion.html

.github/workflows/test.yml

@@ -42,6 +42,11 @@ jobs:
       - name: depends
         run:  bundle install
 
+      # Enable the verbose option in mkmf.rb to print the compiling commands.
+      - name: enable mkmf verbose
+        run:  echo "MAKEFLAGS=V=1" >> $GITHUB_ENV
+        if: runner.os == 'Linux' || runner.os == 'macOS'
+
       - name: compile
         run:  rake compile -- --enable-debug
 
@@ -129,6 +134,10 @@ jobs:
       - name: depends
         run:  bundle install
 
+      - name: enable mkmf verbose
+        run:  echo "MAKEFLAGS=V=1" >> $GITHUB_ENV
+        if: runner.os == 'Linux' || runner.os == 'macOS'
+
       - name: compile
         run:  rake compile -- --enable-debug --with-openssl-dir=$HOME/.openssl/${{ matrix.openssl }}