Macro-based log utility for Neovim

This commit introduces 4 macros (for different log levels) that can be used
to log messages to $HOME/.nvimlog:

 - DLOG: log a debug message (e.g. `DLOG("sum(%d, %d): %d", x, y, sum(x, y));`)
 - ILOG: log some useful information (e.g. `ILOG("Main loop started")`)
 - WLOG: log a warning (e.g. `WLOG("Command not found: %s", command)`)
 - ELOG: log an error (e.g. `ELOG("Out of memory. Exiting.")`)

All these macros are disabled if `NDEBUG` or `DISABLE_LOG` is defined. This
guarantees that a `Release` build won't log anything.

`MIN_LOG_LEVEL` can be defined to reduce the verbosity of the log. The log
levels are:

```
DEBUG_LOG_LEVEL     0
INFO_LOG_LEVEL      1
WARNING_LOG_LEVEL   2
ERROR_LOG_LEVEL     3
```

`MIN_LOG_LEVEL` is 0 by default enabling all levels. If `MIN_LOG_LEVEL` is set
to 2, for example, only warnings and errors will be logged.

That's how the log looks like:

```
     DATETIME   LOG_LEVEL  FUNCTION  LINE  PID      FORMATTED MESSAGE

2014/05/01 23:46:14 [info @ main_loop:582] 44376 - Starting Neovim main loop.
2014/05/01 23:46:31 [info @ main_loop:582] 44400 - Starting Neovim main loop.
```

1 files changed, 57 insertions, 0 deletions

diff --git a/src/log.h b/src/log.h
new file mode 100644
index 0000000000..45c8fdbfd8
--- /dev/null
+++ b/src/log.h
@@ -0,0 +1,57 @@
+#ifndef NEOVIM_LOG_H
+#define NEOVIM_LOG_H
+
+#include <stdbool.h>
+
+#include "func_attr.h"
+
+#define DEBUG_LOG_LEVEL 0
+#define INFO_LOG_LEVEL 1
+#define WARNING_LOG_LEVEL 2
+#define ERROR_LOG_LEVEL 3
+
+bool do_log(int log_level, const char *func_name, int line_num,
+            const char* fmt, ...) FUNC_ATTR_UNUSED;
+
+#define DLOG(...)
+#define ILOG(...)
+#define WLOG(...)
+#define ELOG(...)
+
+// Logging is disabled if NDEBUG or DISABLE_LOG is defined.
+#ifdef NDEBUG
+#  define DISABLE_LOG
+#endif
+
+// MIN_LOG_LEVEL can be defined during compilation to adjust the desired level
+// of logging. DEBUG_LOG_LEVEL is used by default.
+#ifndef MIN_LOG_LEVEL
+#  define MIN_LOG_LEVEL DEBUG_LOG_LEVEL
+#endif
+
+#ifndef DISABLE_LOG
+
+#  if MIN_LOG_LEVEL <= DEBUG_LOG_LEVEL
+#    undef DLOG
+#    define DLOG(...) do_log(DEBUG_LOG_LEVEL, __func__, __LINE__, __VA_ARGS__)
+#  endif
+
+#  if MIN_LOG_LEVEL <= INFO_LOG_LEVEL
+#    undef ILOG
+#    define ILOG(...) do_log(INFO_LOG_LEVEL, __func__, __LINE__, __VA_ARGS__)
+#  endif
+
+#  if MIN_LOG_LEVEL <= WARNING_LOG_LEVEL
+#    undef WLOG
+#    define WLOG(...) do_log(WARNING_LOG_LEVEL, __func__, __LINE__, __VA_ARGS__)
+#  endif
+
+#  if MIN_LOG_LEVEL <= ERROR_LOG_LEVEL
+#    undef ELOG
+#    define ELOG(...) do_log(ERROR_LOG_LEVEL, __func__, __LINE__, __VA_ARGS__)
+#  endif
+
+#endif
+
+#endif  // NEOVIM_LOG_H
+