Link against system-provided libpg_query and xxhash, and respect our CFLAGS.

--- a/ext/pg_query/extconf.rb
+++ b/ext/pg_query/extconf.rb
@@ -7,11 +7,11 @@
 
 $objs = Dir.glob(File.join(__dir__, '*.c')).map { |f| Pathname.new(f).sub_ext('.o').to_s }
 
+$CFLAGS << " #{ENV["CFLAGS"]}"
+$CFLAGS << " -O3" unless $CFLAGS[/-O\d/]
-# -Wno-deprecated-non-prototype avoids warnings on Clang 15.0+, this can be removed in Postgres 16:
-# https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=1c27d16e6e5c1f463bbe1e9ece88dda811235165
-$CFLAGS << " -fvisibility=hidden -O3 -Wall -fno-strict-aliasing -fwrapv -fstack-protector -Wno-unused-function -Wno-unused-variable -Wno-clobbered -Wno-sign-compare -Wno-discarded-qualifiers  -Wno-deprecated-non-prototype -Wno-unknown-warning-option -g"
+$CFLAGS << " -fvisibility=hidden -Wall -fno-strict-aliasing -fwrapv -fstack-protector -Wno-unused-function -Wno-unused-variable -Wno-clobbered -Wno-sign-compare -Wno-discarded-qualifiers"
 
-$INCFLAGS = "-I#{File.join(__dir__, 'include')} " + $INCFLAGS
+$LDFLAGS << " -lpg_query -lxxhash"
 
 SYMFILE =
   if RUBY_PLATFORM =~ /freebsd/
--- a/ext/pg_query/pg_query_ruby.c
+++ b/ext/pg_query/pg_query_ruby.c
@@ -1,5 +1,5 @@
-#include "pg_query.h"
-#include "xxhash/xxhash.h"
+#include <pg_query.h>
+#include <xxhash.h>
 #include <ruby.h>
 
 void raise_ruby_parse_error(PgQueryProtobufParseResult result);