Support ruby-mswin environment on Windows

[Watson1978]

There is some Ruby environment on Windows.

1. MinGW
2. mswin
3. cygwin

The problem in MinGW was fixed in #315
This patch will fix the problem in mswin by adjusting compiler flag values for Visual Studio 2017.

• Before patch

C:\tmp>ruby -v
ruby 2.5.3p105 (2018-10-18 revision 65156) [x64-mswin64_140]

C:\tmp>gem install rmagick-3.0.0.gem
Building native extensions. This could take a while...
ERROR:  Error installing rmagick-3.0.0.gem:
        ERROR: Failed to build gem native extension.

    current directory: C:/usr/lib/ruby/gems/2.5.0/gems/rmagick-3.0.0/ext/RMagick
C:/usr/bin/ruby.exe -r ./siteconf20190303-1888-cmo6ru.rb extconf.rb
checking for X11.lib... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --srcdir=.
        --curdir
        --ruby=C:/usr/bin/ruby
        --with-X11lib
        --without-X11lib
C:/usr/lib/ruby/2.5.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:541:in `try_link0'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:559:in `try_link'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:778:in `try_func'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:1005:in `block in have_library'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:948:in `block in checking_for'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:350:in `block (2 levels) in postpone'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:320:in `open'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:350:in `block in postpone'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:320:in `open'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:346:in `postpone'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:947:in `checking_for'
        from C:/usr/lib/ruby/2.5.0/mkmf.rb:1000:in `have_library'
        from extconf.rb:152:in `configure_compile_options'
        from extconf.rb:17:in `initialize'
        from extconf.rb:452:in `new'
        from extconf.rb:452:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  C:/usr/lib/ruby/gems/2.5.0/extensions/x64-mswin64-140/2.5.0/rmagick-3.0.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in C:/usr/lib/ruby/gems/2.5.0/gems/rmagick-3.0.0 for inspection.
Results logged to C:/usr/lib/ruby/gems/2.5.0/extensions/x64-mswin64-140/2.5.0/rmagick-3.0.0/gem_make.out

• After patch

C:\tmp\new>gem install rmagick-3.0.0.gem
Building native extensions. This could take a while...
Successfully installed rmagick-3.0.0
Parsing documentation for rmagick-3.0.0
Done installing documentation for rmagick after 2 seconds
1 gem installed

C:\tmp\new>ruby -v -e 'require "rmagick"; puts Magick::Version'
ruby 2.5.3p105 (2018-10-18 revision 65156) [x64-mswin64_140]
RMagick 3.0.0

[mockdeep]

Does this have_library do anything here? it looks like we're returning the line below, so not sure whether we need this.

[Watson1978]

Seems that have_library('CORE_RL_magick_') check whether specified library is existed and generate compiler flag to link the library.

[mockdeep]

Ah, I see. I thought maybe it just returned true or false.