'ruby/config.h' file not found

問題

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby.h:33:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/ruby.h:24:10:
fatal error: 'ruby/config.h' file not found
#include "ruby/config.h"
         ^~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/ruby.h:24:10:
note: did not find header 'config.h' in framework 'ruby' (loaded from
'/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks')
1 error generated.
make: *** [generator.o] Error 1

make failed, exit code 2

解決策

ここに書いてあった
https://github.com/orta/cocoapods-keys/issues/198#issuecomment-510909030

自分の場合は以下を実行後、

brew install rbenv ruby-build
echo 'if which rbenv > /dev/null; then eval "$(rbenv init -)"; fi' >> ~/.zshrc 
rbenv install 2.6.3
rbenv global 2.6.3

which gem でパスが以下になっていなかったので、ターミナルを再起動して再度パスを確認後直った。

$ which gem
/Users/<youruser>/.rbenv/shims/gem