Link with system-provided libzstd.

skippable frames are experimental feature that is available only for static
linking, so we have to remove it.

--- a/ext/zstdruby/common.h
+++ b/ext/zstdruby/common.h
@@ -6,7 +6,7 @@
 #include <ruby/thread.h>
 #endif
 #include <stdbool.h>
-#include "./libzstd/zstd.h"
+#include <zstd.h>
 
 static int convert_compression_level(VALUE compression_level_value)
 {
--- a/ext/zstdruby/extconf.rb
+++ b/ext/zstdruby/extconf.rb
@@ -2,24 +2,12 @@
 
 have_func('rb_gc_mark_movable')
 
-$CFLAGS = '-I. -O3 -std=c99 -DZSTD_STATIC_LINKING_ONLY -DZSTD_MULTITHREAD -pthread -DDEBUGLEVEL=0'
-$CPPFLAGS += " -fdeclspec" if CONFIG['CXX'] =~ /clang/
+pkg_config("libzstd")
 
-Dir.chdir File.expand_path('..', __FILE__) do
-  $srcs = Dir['**/*.c', '**/*.S']
-
-    Dir.glob('libzstd/*') do |path|
-        if Dir.exist?(path)
-            $VPATH << "$(srcdir)/#{path}"
-            $INCFLAGS << " -I$(srcdir)/#{path}"
-        end
-    end
-
-end
-
 # add include path to the internal folder
 # $(srcdir) is a root folder, where "extconf.rb" is stored
-$INCFLAGS << " -I$(srcdir) -I$(srcdir)/libzstd"
+$INCFLAGS << " -I$(srcdir)"
+
 # add folder, where compiler can search source files
 $VPATH << "$(srcdir)"
 
--- a/ext/zstdruby/main.c
+++ b/ext/zstdruby/main.c
@@ -2,7 +2,6 @@
 
 VALUE rb_mZstd;
 void zstd_ruby_init(void);
-void zstd_ruby_skippable_frame_init(void);
 void zstd_ruby_streaming_compress_init(void);
 void zstd_ruby_streaming_decompress_init(void);
 
@@ -15,7 +14,6 @@
 
   rb_mZstd = rb_define_module("Zstd");
   zstd_ruby_init();
-  zstd_ruby_skippable_frame_init();
   zstd_ruby_streaming_compress_init();
   zstd_ruby_streaming_decompress_init();
 }
--- a/spec/zstd-ruby_spec.rb
+++ b/spec/zstd-ruby_spec.rb
@@ -13,6 +13,7 @@
 
   describe 'zstd_version' do
     it 'should work' do
+      skip 'do not require specific zstd version'  # XXX-Patch
       expect(Zstd.zstd_version).to eq(10506)
     end
   end