Make the benchmarks group optional.

[junaruga]

I have a suggestion that is to make the benchmark group optional. Right now the installation of the benchmark group by bundle install fails on Ruby >= 2.4. I think this error might prevent a contributor from contributing. I think fixing this issue leads to the contributor's smooth onboarding.

Here is the current GitHub Actions log when trying to install the benchmarks group. You see it fails on Ruby >= 2.4.

$ bundle install --without development
...
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
...
 current directory:
/opt/hostedtoolcache/Ruby/2.4.10/x64/lib/ruby/gems/2.4.0/gems/mysql-2.9.1/ext/mysql_api
make "DESTDIR="
compiling mysql.c
mysql.c: In function ‘stmt_bind_result’:
mysql.c:1320:74: error: ‘rb_cFixnum’ undeclared (first use in this function);
did you mean ‘rb_isalnum’?
else if (argv[i] == rb_cNumeric || argv[i] == rb_cInteger || argv[i] ==
rb_cFixnum)
^~~~~~~~~~
rb_isalnum
...

After making the benchmark optional, we can install the benchmarks group like this when we need it.

$ bundle install --with benchmarks

I am also thinking if we can make the development group optional too. The pros is we can test the bundle install without command line options on the supported platforms in CI. The cons is we have to run bundle install --with development when we need the gems in the development group.

What do you think?

[junaruga]

This PR is also related to #931 . I think making the benchmarks group and the mysql gem optional is more acceptable than removing it, as it still works on Ruby < 2.4.

[junaruga]

@sodabrew ping.

[junaruga]

Thanks!