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

--- a/rmagick.gemspec
+++ b/rmagick.gemspec
@@ -18,11 +18,7 @@
   s.metadata['changelog_uri'] = 'https://github.com/rmagick/rmagick/blob/main/CHANGELOG.md'
   s.metadata['rubygems_mfa_required'] = 'true'
 
-  tracked_files = `git ls-files`.split($OUTPUT_RECORD_SEPARATOR)
-  file_exclusion_regex = /\A(doc|benchmarks|examples|spec|Steepfile)/
-  files = tracked_files.reject { |file| file[file_exclusion_regex] }
-
-  s.files = files
+  s.files = Dir['ext/**/*', 'lib/**/*', 'sig/**/*', '*.gemspec']
   s.require_paths << 'ext'
 
   s.extensions = %w[ext/RMagick/extconf.rb]
@@ -30,5 +26,5 @@
   s.requirements << "ImageMagick #{Magick::MIN_IM_VERSION} or later"
 
   s.add_dependency 'observer', '~> 0.1'
-  s.add_dependency 'pkg-config', '~> 1.4'
+  s.add_development_dependency 'pkg-config', '~> 1.4'
 end