update documentation, don't raise error if library was not found

doc/user/stored-procedures.xml

@@ -758,6 +758,23 @@ lua box.dostring('local f = function(key) t=box.select(0, 0, key); if t ~= nil t
             </programlisting>
         </listitem>
     </varlistentry>
+    <varlistentry>
+        <term>
+            <emphasis role="lua">box.uuid_is_loaded()</emphasis>
+        </term>
+        <listitem>
+            <para>
+                Check if <emphasis>libuuid</emphasis> is available
+                and loaded.
+            </para>
+            <bridgehead renderas="sect4">Example</bridgehead>
+            <programlisting>
+                lua box.uuid_is_loaded()
+                ---
+                 - 1
+                ...
+            </programlisting>
+        </listitem>
 </variablelist>
 
 <variablelist>

mod/box/box_lua_uuid.m

@@ -36,8 +36,12 @@ static box_foo
 static int loaddl_and_call(struct lua_State *L, box_foo f, int raise) {
 	
 	void *libuuid = dlopen("libuuid.so.1", RTLD_LAZY);
-	if (!libuuid)
-		luaL_error(L, "box.uuid(): %s", dlerror());
+	if (!libuuid) {
+		if (raise)
+			return luaL_error(L, "box.uuid(): %s", dlerror());
+		else
+			return 0;
+	}
 
 	uuid_generate = dlsym(libuuid, "uuid_generate");
 	if (!uuid_generate) {
@@ -64,7 +68,6 @@ static int lbox_uuid_loaded(struct lua_State *L) {
 
 	uuid_generate(uuid);
 	lua_pushlstring(L, (char *)uuid, sizeof(uuid_t));
-
 	return 1;
 }