[KERNEL] add support for Linux kernel 4.19 on Ubuntu 18.04 (GCC 7.5)

This patch replaces the __VA_OPT__ varadic macro for the C preprocessor
with a builtin GCC extension for optional varadic arguments. Based on
this change, the dnbd3 Linux kernel module can be also built with GCC
7.5 on a fresh Ubuntu 18.04 with Linux kernel version 4.19.x.

2 files changed, 22 insertions, 8 deletions

diff --git a/README.md b/README.md
index a46423e..1e60477 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,7 @@ The dnbd3 components can be built for the following Linux kernel versions and Un
 
   - Archlinux with **Linux kernel 5.9.x** or **5.4.x**
   - Ubuntu 20.04 with **Linux kernel 5.4.x**
+  - Ubuntu 18.04 with **Linux kernel 4.19.x**
   - CentOS 8 with **Linux kernel 4.18.x**
   - FreeBSD 12.1 (only user space programs, eg. dnbd3-server)
 
@@ -50,6 +51,19 @@ apt-get install git \
 
 Note that `afl` is not available on Ubuntu 20.04 and should be built from the [original sources](https://github.com/google/AFL).
 
+#### Ubuntu 18.04 with Linux kernel 4.19.x
+```shell
+apt-get install git \
+                make \
+                cmake \
+                gcc \
+                linux-headers-generic \
+				libfuse-dev \
+				libjansson-dev \
+                afl \
+                rpm
+```
+
 #### CentOS 8 with Linux kernel 4.18.x
 ```shell
 yum install git \
diff --git a/src/kernel/net.c b/src/kernel/net.c
index a044be2..a0444d2 100644
--- a/src/kernel/net.c
+++ b/src/kernel/net.c
@@ -55,24 +55,24 @@
  */
 #define __dnbd3_dev_dbg_host(dev, host, fmt, ...) do { \
 	if ((host).type == HOST_IP4) { \
-		dev_dbg(dnbd3_device_to_dev((dev)), "(%pI4:%d): "   fmt, (host).addr, (int)ntohs((host).port) __VA_OPT__(,) __VA_ARGS__); \
+		dev_dbg(dnbd3_device_to_dev((dev)), "(%pI4:%d): "   fmt, (host).addr, (int)ntohs((host).port), ##__VA_ARGS__); \
 	} else { \
-		dev_dbg(dnbd3_device_to_dev((dev)), "([%pI6]:%d): " fmt, (host).addr, (int)ntohs((host).port) __VA_OPT__(,) __VA_ARGS__); \
+		dev_dbg(dnbd3_device_to_dev((dev)), "([%pI6]:%d): " fmt, (host).addr, (int)ntohs((host).port), ##__VA_ARGS__); \
 	} \
 } while (0)
 
 #define __dnbd3_dev_err_host(dev, host, fmt, ...) do { \
 	if ((host).type == HOST_IP4) { \
-		dev_err(dnbd3_device_to_dev((dev)), "(%pI4:%d): "   fmt, (host).addr, (int)ntohs((host).port) __VA_OPT__(,) __VA_ARGS__); \
+		dev_err(dnbd3_device_to_dev((dev)), "(%pI4:%d): "   fmt, (host).addr, (int)ntohs((host).port), ##__VA_ARGS__); \
 	} else { \
-		dev_err(dnbd3_device_to_dev((dev)), "([%pI6]:%d): " fmt, (host).addr, (int)ntohs((host).port) __VA_OPT__(,) __VA_ARGS__); \
+		dev_err(dnbd3_device_to_dev((dev)), "([%pI6]:%d): " fmt, (host).addr, (int)ntohs((host).port), ##__VA_ARGS__); \
 	} \
 } while (0)
 
-#define dnbd3_dev_dbg_host_cur(dev, fmt, ...) __dnbd3_dev_dbg_host((dev), (dev)->cur_server.host,     fmt __VA_OPT__(,) __VA_ARGS__)
-#define dnbd3_dev_err_host_cur(dev, fmt, ...) __dnbd3_dev_err_host((dev), (dev)->cur_server.host,     fmt __VA_OPT__(,) __VA_ARGS__)
-#define dnbd3_dev_dbg_host_alt(dev, fmt, ...) __dnbd3_dev_dbg_host((dev), (dev)->alt_servers[i].host, fmt __VA_OPT__(,) __VA_ARGS__)
-#define dnbd3_dev_err_host_alt(dev, fmt, ...) __dnbd3_dev_err_host((dev), (dev)->alt_servers[i].host, fmt __VA_OPT__(,) __VA_ARGS__)
+#define dnbd3_dev_dbg_host_cur(dev, fmt, ...) __dnbd3_dev_dbg_host((dev), (dev)->cur_server.host,     fmt, ##__VA_ARGS__)
+#define dnbd3_dev_err_host_cur(dev, fmt, ...) __dnbd3_dev_err_host((dev), (dev)->cur_server.host,     fmt, ##__VA_ARGS__)
+#define dnbd3_dev_dbg_host_alt(dev, fmt, ...) __dnbd3_dev_dbg_host((dev), (dev)->alt_servers[i].host, fmt, ##__VA_ARGS__)
+#define dnbd3_dev_err_host_alt(dev, fmt, ...) __dnbd3_dev_err_host((dev), (dev)->alt_servers[i].host, fmt, ##__VA_ARGS__)
 
 static inline dnbd3_server_t *get_free_alt_server(dnbd3_device_t * const dev)
 {