y.tab.c: remove auto generated `y.tab.c` from repository.

It used to be generated in the build, but `bison` required to generation
was too hard to install correctly on MacOS and Windows (version mismatch
and package maneger issues). This is retry.

.github/workflows/build.yml

@@ -123,7 +123,7 @@ jobs:
           --only-site --site http://mirrors.kernel.org/sourceware/cygwin/
           --root ${{ env.cygwin-root }}
           --local-package-dir ${{ env.package-dir }}
-          --packages gcc-core,gcc-g++,ruby
+          --packages gcc-core,gcc-g++,ruby,bison
       - name: Set PATH for Cygwin
         run: |
           echo '::set-env name=PATH::${{ env.cygwin-root }}\bin;${{ env.cygwin-root }}\usr\bin'
@@ -151,5 +151,6 @@ jobs:
       - name: Build and test
         shell: cmd
         run: |
+          choco install winflexbison3
           call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Auxiliary\Build\vcvars64.bat"
           rake -m test:build && rake test:run

.gitignore

@@ -23,6 +23,7 @@ compile_commands.json
 cscope.files
 cscope.out
 tags
+y.tab.c
 
 /.git
 /bin

appveyor.yml

@@ -38,6 +38,8 @@ init:
   - set PATH=C:\Ruby26-x64\bin;%PATH%
   - ruby --version
 
+install:
+  - choco install winflexbison3
 
 build_script:
   - set MRUBY_CONFIG=ci/msvc