I cannot to compile this extension against master branch

I got an error when using it with timescaledb.

PostgreSQL Version: 12.3
TimescaleDB Version: 1.7.0
OS Version: CentOS 7.6
Kernel Version: 5.7.3-1.el7.elrepo.x86_64

CREATE EXTENSION IF NOT EXISTS timescaledb CASCADE;
CREATE EXTENSION IF NOT EXISTS pg_cryogen;

CREATE TABLE "public"."log" (
"time" timestamptz(6) NOT NULL,
"device_id" TEXT NOT NULL,
"message_id" integer NOT NULL,
"project_id" TEXT NOT NULL,
"createtime" timestamp(6) NOT NULL,
"service_type" integer NOT NULL,
"message_type" integer NOT NULL,
"data_type" TEXT NOT NULL,
"metrics_name" TEXT NOT NULL,
"metrics_value" TEXT NOT NULL
) using pg_cryogen;

SELECT create_hypertable('log', 'time', chunk_time_interval => interval '1 hours', create_default_indexes => false);
ERROR: only heap AM is supported
SQL State: 0A000

Please let me know if there is a problem with my usage. Thanks~

fix

diff --git a/compression.h b/compression.h
index 7d875c5..865b7ce 100644
--- a/compression.h
+++ b/compression.h
@@ -10,17 +10,17 @@ typedef enum
     COMP_ZSTD
 } CompressionMethod;
 
-int compression_method_guc;
-int lz4_acceleration_guc;
-int zstd_compression_level_guc;
+extern int compression_method_guc;
+extern int lz4_acceleration_guc;
+extern int zstd_compression_level_guc;
 
-char *cryo_compress(CompressionMethod method,
+extern char *cryo_compress(CompressionMethod method,
                     const char *data,
                     Size *compressed_size);
-bool cryo_decompress(CompressionMethod method,
+extern bool cryo_decompress(CompressionMethod method,
                      const char *compressed,
                      Size compressed_size,
                      char *out);
-void cryo_define_compression_gucs(void);
+extern void cryo_define_compression_gucs(void);
 
 #endif /* __COMPRESSION_H__ */

I tested insert 1M rows

create table test(a int);
create table test2 (like test) using pg_cryogen;
insert into test select random()*100000 from generate_series(1,1000000);

with

set pg_cryogen.zstd_compression_level=22;

the statement

insert into test2 select * from test;

requires about 4GB RAM (original table has 35MB, compressed table has 27MB).