Introduce colored fatal error messages

Introduces the fatal() function that
1) Check whether stderr is a terminal, and sets red text color if it is
2) Prints the message
3) Exits the app

.gitignore

@@ -3,6 +3,7 @@
 
 # generated from MANPAGE.md
 /earlyoom.1
+/earlyoom.1.gz
 
 # generated service files
 /earlyoom.service

msg.c

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MIT
+
+#include <stdlib.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <unistd.h>
+
+// Print message to stderr and exit with "code".
+// Example: fatal(6, "could not compile regexp '%s'\n", regex_str);
+void fatal(int code, char* fmt, ...)
+{
+    char* red = "";
+    char* reset = "";
+    if (isatty(fileno(stderr))) {
+        red = "\033[31m";
+        reset = "\033[0m";
+    }
+    char fmt2[100];
+    snprintf(fmt2, sizeof(fmt2), "%sfatal: %s%s", red, fmt, reset);
+    va_list args;
+    va_start(args, fmt); // yes fmt, NOT fmt2!
+    vfprintf(stderr, fmt2, args);
+    va_end(args);
+    exit(code);
+}

msg.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: MIT */
+#ifndef MSG_H
+#define MSG_H
+
+void fatal(int code, char* fmt, ...);
+
+#endif