zabbix agent module compiled on Alpine causes symbol not found

[chace20]

When I build a .so module in Alpine OS, I found that always some symbol not found caused compile fail. But it's normal in other platform(centOS, ubuntu).

```
12158:20180913:074631.818 In zbx_load_modules()
12158:20180913:074631.818 loading module "/home/zabbix/src/modules/zabbix_module_docker/zabbix_module_docker.so"
12158:20180913:074631.822 cannot load module "zabbix_module_docker.so": Error relocating /home/zabbix/src/modules/zabbix_module_docker/zabbix_module_docker.so: zbx_strcpy_alloc: symbol not found
12158:20180913:074631.822 End of zbx_load_modules():FAIL
12158:20180913:074631.822 loading modules failed, exiting...
```

Then I found it's because zabbix agent is compiled by static in Alpine, while compiled by dynamic in other platform. The`config.log` in Alpine is below. It seems `execinfo.h` is missed and check function is not work cause `-rdynamic` linking option is off.
```
configure:7558: checking for -rdynamic linking option
configure:7573: gcc -o conftest -g -O2 -rdynamic conftest.c -lm >&5
conftest.c:108:22: fatal error: execinfo.h: No such file or directory
compilation terminated.
configure:7573: $? = 1
configure: failed program was:
| /* confdefs.h */
| #define PACKAGE_NAME "Zabbix"
| #define PACKAGE_TARNAME "zabbix"
| #define PACKAGE_VERSION "3.4.12"
| #define PACKAGE_STRING "Zabbix 3.4.12"
......
| #define HAVE_PROC_CPUINFO 1
| #define HAVE_PROC_LOADAVG 1
| #define HAVE_PROC_NET_DEV 1
| /* end confdefs.h. */
| #include <execinfo.h>
| int
| main ()
| {
| void *bcktrc[6];
| (void)backtrace(bcktrc, 6);
| return 0;
| }
configure:7579: result: no
```

After try, I found I can export a new environment variable LDFLAGS, and then run configure to force dynamic compile.
```
export LDFLAGS="-rdynamic"
./configure --enable-agent
```

Could zabbix team add a tip in [loadablemodules documentation](https://www.zabbix.com/documentation...oadablemodules), or do something else?