Ruby/Tk インストールできないです

Ruby/Tkをgem install tkでインストールしようとしたら、以下のエラーが出ました。解決策を調べてもhitせず、お力をお借りできれば幸いです。

Fail to find [tclConfig.sh, tkConfig.sh]が肝だと思うのですが。。

発生している問題・エラーメッセージ

~ $ gem install tk
Fetching tk-0.3.0.gem
Building native extensions. This could take a while...
ERROR:  Error installing tk:
	ERROR: Failed to build gem native extension.

    current directory: /Users/****/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/tk-0.3.0/ext/tk
/Users/****/.rbenv/versions/2.6.5/bin/ruby -I /Users/****/.rbenv/versions/2.6.5/lib/ruby/2.6.0 -r ./siteconf20201215-2090-g0qb3m.rb extconf.rb
check functions.checking for ruby_native_thread_p() in ruby.h... yes
checking for rb_errinfo() in ruby.h... yes
checking for rb_hash_lookup() in ruby.h... yes
checking for rb_proc_new() in ruby.h... yes
checking for rb_sourcefile() in ruby.h... yes
checking for rb_thread_alive_p() in ruby.h... no
checking for rb_thread_check_trap_pending() in ruby.h... yes
checking for ruby_enc_find_basename() in ruby.h... yes
check libraries.checking for t_open() in -lnsl... no
checking for socket() in -lsocket... no
checking for dlopen() in -ldl... yes
checking for log() in -lm... yes
Use ActiveTcl libraries (if available).
Search tclConfig.sh and tkConfig.sh.
Fail to find [tclConfig.sh, tkConfig.sh]
Use X11 libraries (or use TK_XINCLUDES/TK_XLIBSW information on tkConfig.sh).
checking for XOpenDisplay() in -lX11... no
*** 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:
	~~~略~~~

Warning:: cannot find X11 library. tcltklib will not be compiled (tcltklib is disabled on your Ruby. That is, Ruby/Tk will not work). Please check configure options. If your Tcl/Tk don't require X11, please try --without-X11.
Can't find X11 libraries. 
So, can't make tcltklib.so which is required by Ruby/Tk.

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

  /Users/****/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-19/2.6.0/tk-0.3.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/****/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/tk-0.3.0 for inspection.
Results logged to /Users/****/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-19/2.6.0/tk-0.3.0/gem_make.out

試したこと

mdfindでtclConfig.sh, tkConfig.shを探しましたが存在しました。

/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5/tkConfig.sh

/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Tcl.framework/Versions/8.5/tclConfig.sh

また、以下のURLを参考に、以下のように実行し、再度インストールしたら、エラーが変わりました。
tk/tclライブラリが見つからないようですが、適切なライブラリがどこにあるのか、わかりません。。

https://qiita.com/ftakao2007/items/230050be7351db49383d
https://github.com/ruby/tk/blob/master/README.tcltklib
http://www.mit.edu/afs.new/sipb/service/restore/ruby-lang/src/ruby-1.8.4/ext/tk/README.tcltklib

$ RUBY_CONFIGURE_OPTS="--with-tcltkversion=8.5 \
--with-tcl-lib=/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Tcl.framework/Versions/8.5 \
--with-tk-lib=/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5 \
--enable-pthread --enable-shared" rbenv install 2.6.5

$ gem install tk
Building native extensions. This could take a while...
ERROR:  Error installing tk:
	ERROR: Failed to build gem native extension.

    current directory: /Users/****/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/tk-0.3.0/ext/tk
/Users/****/.rbenv/versions/2.6.5/bin/ruby -I /Users/****/.rbenv/versions/2.6.5/lib/ruby/2.6.0 -r ./siteconf20201215-15693-14ro7nf.rb extconf.rb
Configure options for Ruby/Tk may be updated.
So, delete files which depend on old configs.
check functions.checking for ruby_native_thread_p() in ruby.h... yes
checking for rb_errinfo() in ruby.h... yes
checking for rb_hash_lookup() in ruby.h... yes
checking for rb_proc_new() in ruby.h... yes
checking for rb_sourcefile() in ruby.h... yes
checking for rb_thread_alive_p() in ruby.h... no
checking for rb_thread_check_trap_pending() in ruby.h... yes
checking for ruby_enc_find_basename() in ruby.h... yes
check libraries.checking for t_open() in -lnsl... no
checking for socket() in -lsocket... no
checking for dlopen() in -ldl... yes
checking for log() in -lm... yes
Specified Tcl/Tk version is ["8.5", "8.5"]
Use ActiveTcl libraries (if available).
Search tclConfig.sh and tkConfig.sh.
Fail to find [tclConfig.sh, tkConfig.sh]
Search tcl.h
checking for tcl.h... yes
Search tk.h
checking for tk.h... yes
Search Tcl library.checking for Tcl_FindExecutable() in -ltclstub8.5... no
.checking for Tcl_FindExecutable() in -ltclstub8.5... no
.checking for Tcl_FindExecutable() in -ltclstub8.5... no
.checking for Tcl_FindExecutable() in -ltclstub8.5... no
*** 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:
	~~~略~~~

Warning:: cannot find Tcl library. tcltklib will not be compiled (tcltklib is disabled on your Ruby. That is, Ruby/Tk will not work). Please check configure options.

Can't find proper Tcl/Tk libraries. So, can't make tcltklib.so which is required by Ruby/Tk.
If you have Tcl/Tk libraries on your environment, you may be able to use them with configure options (see ext/tk/README.tcltklib).
At present, Tcl/Tk8.6 is not supported. Although you can try to use Tcl/Tk8.6 with configure options, it will not work correctly. I recommend you to use Tcl/Tk8.5 or 8.4.

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

  /Users/****/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-19/2.6.0/tk-0.3.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/****/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/tk-0.3.0 for inspection.
Results logged to /Users/****/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-19/2.6.0/tk-0.3.0/gem_make.out