Add compat/fparseln() for non-BSD systems

Linux and friends don't natively have fparseln() so add it to compat/ and
ensure autotools can pick it up.

compat.h

@@ -239,6 +239,10 @@ int		 vasprintf(char **, const char *, va_list);
 char		*fgetln(FILE *, size_t *);
 #endif
 
+#ifndef HAVE_FPARSELN
+char		*fparseln(FILE *, size_t *, size_t *, const char *, int);
+#endif
+
 #ifndef HAVE_SETENV
 /* setenv.c */
 int		 setenv(const char *, const char *, int);