client: add a field with Curl version

```
tarantool> require("smtp")._CURL_VERSION
---
- 7.81.0
...
```

Needed for #70

Author: ligurio

smtp/init.lua

@@ -277,7 +277,7 @@ curl_mt = {
 --
 -- Export
 --
-local this_module = { new = smtp_new, }
+local this_module = { new = smtp_new, _CURL_VERSION = driver._CURL_VERSION }
 
 package.loaded['smtp.client'] = this_module
 return this_module

smtp/lib.c

@@ -255,6 +255,15 @@ luaT_smtpc_cleanup(lua_State *L)
 	return 2;
 }
 
+static int
+luaT_smtpc_version(lua_State *L)
+{
+	unsigned int major, minor, patch;
+	smtpc_get_curl_version(&major, &minor, &patch);
+	lua_pushfstring(L, "%u.%u.%u", major, minor, patch);
+	return 1;
+}
+
 /*
  * }}}
  */
@@ -292,6 +301,11 @@ luaopen_smtp_lib(lua_State *L)
 	luaL_register(L, NULL, Client);
 	lua_pop(L, 1);
 	luaL_register(L, "smtp.client.driver", Module);
+
+	lua_pushliteral(L, "_CURL_VERSION");
+	luaT_smtpc_version(L);
+	lua_rawset(L, -3);
+
 	return 1;
 }
 

smtp/smtpc.c

@@ -150,6 +150,15 @@ bind_libcurl_functions(const char *libname, void *libcurl_handle)
 
 #undef load_func
 
+void
+smtpc_get_curl_version(unsigned int *major, unsigned int *minor, unsigned int *patch)
+{
+	curl_version_info_data *info = curl_version_info(CURLVERSION_NOW);
+	*major = (info->version_num >> 16) & 0xff;
+	*minor = (info->version_num >> 8) & 0xff;
+	*patch = info->version_num & 0xff;
+}
+
 int
 smtpc_init(void)
 {

smtp/smtpc.h

@@ -271,4 +271,16 @@ smtpc_execute(struct smtpc_request *req, double timeout);
 
 /** Request }}} */
 
+/* {{{ Version */
+
+/**
+ * Get a Curl library version.
+ *
+ * @see https://curl.se/libcurl/c/curl_version_info.html
+ */
+void
+smtpc_get_curl_version(unsigned int *major, unsigned int *minor, unsigned int *patch);
+
+/* Version }}} */
+
 #endif /* TARANTOOL_SMTPC_H_INCLUDED */