Global CONFIG_FILE variable defined without extern in header file

  Since GCC 10.1.0 (2020-05-07) default compilation options to use -fno-common, which means that by default, the code above no longer links unless you override the default with -fcommon

src/umonitor.c

@@ -93,6 +93,8 @@ static const struct option long_options[] = {
 	{"force-load", no_argument, &force_load, 1}
 };
 
+/* Definition checked against declaration in umonitor.h */
+char* CONFIG_FILE;
 
 void umon_print(const char *format, ...)
 {
@@ -127,6 +129,7 @@ void print_state(const char *format, ...)
 
 /*! Logic for parsing options here*/
 
+
 int main(int argc, char **argv)
 {
 	home_directory = getenv("HOME");
@@ -147,6 +150,7 @@ int main(int argc, char **argv)
 	print_info();
 
 	screen_class_constructor(&screen_o);
+
 	CONFIG_FILE =
 	    umalloc((strlen(home_directory) + strlen(conf_location))+1);
 	strcpy(CONFIG_FILE, home_directory);

src/umonitor.h

@@ -1,5 +1,5 @@
 // Global variables
-char *CONFIG_FILE;		/*!< Configuration file path name */
+extern char *CONFIG_FILE;		/*!< Configuration file path name */
 
 // Some helper functions
 void fetch_edid(xcb_randr_output_t * output_p, screen_class * screen_t_p,