Allow debug prints without __VA_ARGS__

__VA_ARGS__ are frustrating in C. Even for their main purpose (printf),
they fall short in that they don't have a _portable_ way to have zero
arguments after the format string in a printf call.

Even if we detect compilers and use ##__VA_ARGS__ where available, GCC
emits a warning with -pedantic that is _impossible_ to explicitly
disable.

This commit contains the best solution we can think of. A bit of
indirection that adds a hidden "%s" % "" to the end of the format
string. This solution does not work everywhere as it has a runtime
cost, but it is hopefully ok for debug statements.

lfs_util.h

@@ -50,31 +50,35 @@ extern "C"
 
 // Logging functions
 #ifdef LFS_YES_TRACE
-#define LFS_TRACE(fmt, ...) \
-    printf("lfs_trace:%d: " fmt "\n", __LINE__, __VA_ARGS__)
+#define LFS_TRACE_(fmt, ...) \
+    printf("lfs_trace:%d: " fmt "%s\n", __LINE__, __VA_ARGS__)
+#define LFS_TRACE(...) LFS_TRACE_(__VA_ARGS__, "")
 #else
-#define LFS_TRACE(fmt, ...)
+#define LFS_TRACE(...)
 #endif
 
 #ifndef LFS_NO_DEBUG
-#define LFS_DEBUG(fmt, ...) \
-    printf("lfs_debug:%d: " fmt "\n", __LINE__, __VA_ARGS__)
+#define LFS_DEBUG_(fmt, ...) \
+    printf("lfs_debug:%d: " fmt "%s\n", __LINE__, __VA_ARGS__)
+#define LFS_DEBUG(...) LFS_DEBUG_(__VA_ARGS__, "")
 #else
-#define LFS_DEBUG(fmt, ...)
+#define LFS_DEBUG(...)
 #endif
 
 #ifndef LFS_NO_WARN
-#define LFS_WARN(fmt, ...) \
-    printf("lfs_warn:%d: " fmt "\n", __LINE__, __VA_ARGS__)
+#define LFS_WARN_(fmt, ...) \
+    printf("lfs_warn:%d: " fmt "%s\n", __LINE__, __VA_ARGS__)
+#define LFS_WARN(...) LFS_WARN_(__VA_ARGS__, "")
 #else
-#define LFS_WARN(fmt, ...)
+#define LFS_WARN(...)
 #endif
 
 #ifndef LFS_NO_ERROR
-#define LFS_ERROR(fmt, ...) \
-    printf("lfs_error:%d: " fmt "\n", __LINE__, __VA_ARGS__)
+#define LFS_ERROR_(fmt, ...) \
+    printf("lfs_error:%d: " fmt "%s\n", __LINE__, __VA_ARGS__)
+#define LFS_ERROR(...) LFS_ERROR_(__VA_ARGS__, "")
 #else
-#define LFS_ERROR(fmt, ...)
+#define LFS_ERROR(...)
 #endif
 
 // Runtime assertions