Add GFX2_Log() function

src/gfx2log.h

@@ -0,0 +1,39 @@
+/* vim:expandtab:ts=2 sw=2:
+*/
+/*  Grafx2 - The Ultimate 256-color bitmap paint program
+
+    Copyright 2018 Thomas Bernard
+    Copyright 1996-2001 Sunset Design (Guillaume Dorme & Karl Maritaud)
+
+    Grafx2 is free software; you can redistribute it and/or
+    modify it under the terms of the GNU General Public License
+    as published by the Free Software Foundation; version 2
+    of the License.
+
+    Grafx2 is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with Grafx2; if not, see <http://www.gnu.org/licenses/>
+*/
+#ifndef GFX2LOG_H_DEFINED
+#define GFX2LOG_H_DEFINED
+
+#include <stdarg.h>
+
+typedef enum {
+  GFX2_ERROR = 0,
+  GFX2_WARNING,
+  GFX2_INFO,
+  GFX2_DEBUG
+} GFX2_Log_priority_T;
+
+extern GFX2_Log_priority_T GFX2_verbosity_level;
+
+extern void GFX2_Log(GFX2_Log_priority_T priority, const char * fmt, ...);
+
+extern void GFX2_LogV(GFX2_Log_priority_T priority, const char * fmt, va_list ap);
+
+#endif