Add --enable-static option to link icu4c libraries statically

Homebrew in macOS updates the `icu4c` library quite often, which often
breaks this gem because the dyanmically-loaded library no longer
exists.

To avoid this hassle, this gem can be compiled with `--enable-static`
so that the resulting `charlock_holmes.bundle` will have all the
required libraries statically linked.

ext/charlock_holmes/extconf.rb

@@ -51,4 +51,52 @@
 $CFLAGS << ' -Wall -funroll-loops'
 $CFLAGS << ' -Wextra -O0 -ggdb3' if ENV['DEBUG']
 
+def libflag_to_filename(ldflag)
+  case ldflag
+  when /\A-l(.+)/
+    "lib#{Regexp.last_match(1)}.#{$LIBEXT}"
+  end
+end
+
+def resolve_static_library(libflag, dirs)
+  filename = libflag_to_filename(libflag)
+
+  dir = dirs.find { |path| File.exist?(File.join(path, filename)) }
+
+  raise "Unable to find #{filename} in #{dirs}" unless dir
+
+  File.join(dir, filename)
+end
+
+def substitute_static_libs(packages)
+  # First, find all the -l<lib> flags added by pkg-config. We want to drop
+  # these dynamically linked libraries and substitute them with the static libraries.
+  libflags = packages.map do |pkg|
+    pkg_config(pkg, 'libs-only-l')&.strip&.split(' ')
+  end.flatten.uniq
+
+  # To find where the static libraries live, we need to search the
+  # library paths given by the -L flag from pkg-config.
+  lib_paths = packages.map do |pkg|
+    include_path = pkg_config(pkg, 'libs-only-L')&.strip
+    include_path&.split(' ').map { |lib| lib.gsub(/^-L/, '') }
+  end.flatten.uniq
+
+  # Drop the -l<lib> flags and add in the static libraries.
+  new_libs = $libs.shellsplit
+  new_libs.reject! { |arg| libflags.include?(arg) }
+  libflags.each { |flag| new_libs << resolve_static_library(flag, lib_paths) }
+  $libs = new_libs.uniq.shelljoin
+end
+
+static_p = enable_config('static', false)
+message "Static linking is #{static_p ? 'enabled' : 'disabled'}.\n"
+
+if static_p
+  $CXXFLAGS << ' -fPIC'
+  ENV['PKG_CONFIG_ALLOW_SYSTEM_LIBS'] = '1'
+
+  substitute_static_libs(%w[icu-i18n icu-io icu-uc])
+end
+
 create_makefile 'charlock_holmes/charlock_holmes'