Build error: library not found for -lGBCli

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Hi,

Thanks for the wonderful project.

I am a novice in macosx programming. I was following your instructions to build macdown and came across the following error.

````
ld: warning: directory not found for option '-L/Users/XXXX/Library/Developer/Xcode/DerivedData/MacDown-eqwszngivaluxugduwftzfyizdtk/Build/Products/Debug/GBCli'
ld: library not found for -lGBCli
clang: error: linker command failed with exit code 1 (use -v to see invocation)
````

There was no error during the setup as follows. So I am curious what should I try to get it built. I appreciate any guidance in advance. Thank you.

````
$ bundle exec pod install
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/universal-darwin18/rbconfig.rb:215: warning: Insecure world writable dir /Users/XXXX/.local in PATH, mode 040777
Analyzing dependencies
Downloading dependencies
Using GBCli (1.1)
Using JJPluralForm (2.1)
Using LibYAML (0.1.4)
Using M13OrderedDictionary (1.1.0)
Using MASPreferences (1.3)
Using PAPreferences (0.5)
Using Sparkle (1.18.1)
Using handlebars-objc (1.4.5)
Using hoedown (3.0.7)
Generating Pods project
Integrating client project
Sending stats
Pod installation complete! There are 9 dependencies from the Podfile and 9 total pods installed.
````

If you need any more info, I will provide it.

Start by reproducing the macOS build after running `bundle exec pod install`, then inspect the generated Pods project and the GBCli dependency referenced by the linker. Done means the MacDown build resolves GBCli and completes without the missing-directory or missing-library errors.

Written by the indexing model from the issue text.