Example code doesn't compile on 1.8.13

[AverageGuy]

/home/jwl/sketchbook/libraries/telnetspy-master/TelnetSpy.cpp: In member function 'void TelnetSpy::setDebugOutput(bool)':
/home/jwl/sketchbook/libraries/telnetspy-master/TelnetSpy.cpp:406:42: error: invalid conversion from 'void*' to 'fp_putc_t {aka void ()(char)}' [-fpermissive]
os_install_putc1((void) TelnetSpy_putc); // Set system printing (os_printf) to TelnetSpy
^
In file included from /home/jwl/.arduino15/packages/esp8266/hardware/esp8266/2.7.1/tools/sdk/include/os_type.h:28:0,
from /home/jwl/.arduino15/packages/esp8266/hardware/esp8266/2.7.1/tools/sdk/include/user_interface.h:28,
from /home/jwl/sketchbook/libraries/telnetspy-master/TelnetSpy.cpp:12:
/home/jwl/.arduino15/packages/esp8266/hardware/esp8266/2.7.1/tools/sdk/include/ets_sys.h:219:6: error: initializing argument 1 of 'void ets_install_putc1(fp_putc_t)' [-fpermissive]
void ets_install_putc1(fp_putc_t routine);
^
/home/jwl/sketchbook/libraries/telnetspy-master/TelnetSpy.cpp:415:50: error: invalid conversion from 'void*' to 'fp_putc_t {aka void ()(char)}' [-fpermissive]
os_install_putc1((void) TelnetSpy_ignore_putc); // Ignore system printing
^
In file included from /home/jwl/.arduino15/packages/esp8266/hardware/esp8266/2.7.1/tools/sdk/include/os_type.h:28:0,
from /home/jwl/.arduino15/packages/esp8266/hardware/esp8266/2.7.1/tools/sdk/include/user_interface.h:28,
from /home/jwl/sketchbook/libraries/telnetspy-master/TelnetSpy.cpp:12:
/home/jwl/.arduino15/packages/esp8266/hardware/esp8266/2.7.1/tools/sdk/include/ets_sys.h:219:6: error: initializing argument 1 of 'void ets_install_putc1(fp_putc_t)' [-fpermissive]
void ets_install_putc1(fp_putc_t routine);
^
Multiple libraries were found for "ArduinoOTA.h"
Used: /home/jwl/.arduino15/packages/esp8266/hardware/esp8266/2.7.1/libraries/ArduinoOTA
Not used: /home/jwl/sketchbook/libraries/ArduinoOTA
exit status 1
Error compiling for board Generic ESP8266 Module.

[AverageGuy]

So I decided to hack away at the problem and came up with this solution. It now compiles but I havn't tested it to see if it works. The compile errors go away:
--- TelnetSpy.cpp.orig 2019-06-11 03:39:16.000000000 -0400
+++ TelnetSpy.cpp 2020-07-01 08:28:32.772613248 -0400
@@ -403,7 +403,7 @@
if (debugOutput) {
actualObject = this;
#ifdef ESP8266

•   os_install_putc1((void*) TelnetSpy_putc);  // Set system printing (os_printf) to TelnetSpy
  

•   os_install_putc1( TelnetSpy_putc);  // Set system printing (os_printf) to TelnetSpy
    system_set_os_print(true);
  

#else // ESP32
// ToDo: How can be done this for ESP32 ?
@@ -412,7 +412,7 @@
if (actualObject == this) {
#ifdef ESP8266
system_set_os_print(false);

•   	os_install_putc1((void*) TelnetSpy_ignore_putc); // Ignore system printing
  

•   	os_install_putc1( TelnetSpy_ignore_putc); // Ignore system printing
  

#else // ESP32
// ToDo: How can be done this for ESP32 ?
#endif

Results from compile:

Executable segment sizes:
RODATA : 1540 ) / 81920 - constants (global, static) in RAM/HEAP
DATA : 1276 ) - initialized variables (global, static) in RAM/HEAP
IRAM : 27856 / 32768 - code in IRAM (ICACHE_RAM_ATTR, ISRs...)
BSS : 25576 ) - zeroed variables (global, static) in RAM/HEAP
IROM : 285660 - code in flash (default or ICACHE_FLASH_ATTR)
Sketch uses 316332 bytes (33%) of program storage space. Maximum is 958448 bytes.
Global variables use 28392 bytes (34%) of dynamic memory, leaving 53528 bytes for local variables. Maximum is 81920 bytes.

[sebitnt]

After applying the above changes it is working fine for me. The author should update the code. Thank you!