About pkg-config dependency see https://github.com/rmagick/rmagick/commit/65c18a1eaf8f5b81b806d02b5bb42c9a013107bb#r90271637

--- a/rmagick.gemspec
+++ b/rmagick.gemspec
@@ -13,7 +13,7 @@
   s.homepage = 'https://github.com/rmagick/rmagick'
   s.license = 'MIT'
 
-  tracked_files = `git ls-files`.split($OUTPUT_RECORD_SEPARATOR)
+  tracked_files = Dir['deprecated/*', 'ext/**/*', 'lib/**/*', '*.gemspec']
   file_exclusion_regex = %r{\A(doc|benchmarks|examples|spec|lib/rvg/to_c.rb)}
   files = tracked_files.reject { |file| file[file_exclusion_regex] }
 
@@ -24,7 +24,7 @@
   s.required_ruby_version = ">= #{Magick::MIN_RUBY_VERSION}"
   s.requirements << "ImageMagick #{Magick::MIN_IM_VERSION} or later"
 
-  s.add_runtime_dependency 'pkg-config', '~> 1.4'
+  s.add_development_dependency 'pkg-config', '~> 1.4'
 
   s.add_development_dependency 'pry', '~> 0.14'
   s.add_development_dependency 'rake-compiler', '~> 1.0'