Error - Package appindicator3-0.1 was not found in the pkg-config search path.

@PC20240309:~/indicator-netspeed$ make
Package appindicator3-0.1 was not found in the pkg-config search path.
Perhaps you should add the directory containing `appindicator3-0.1.pc'
to the PKG_CONFIG_PATH environment variable
No package 'appindicator3-0.1' found
gcc indicator-netspeed.c -g -Wall -Wfatal-errors -std=c99 -o indicator-netspeed
indicator-netspeed.c:17:10: fatal error: gtk/gtk.h: Datei oder Verzeichnis nicht gefunden
17 | #include
| ^~~~~~~~~~~
compilation terminated.
make: *** [Makefile:7: indicator-netspeed] Fehler 1

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Unfortunately, I am not an experienced Ubuntu user and would like to use the function according to the following instructions.

https://wiki.ubuntuusers.de/Netspeed/

Start with the Makefile target that invokes gcc and the dependency errors reported for appindicator3-0.1 and gtk/gtk.h. Check the Ubuntu installation instructions linked in the issue and confirm which development packages are required; the issue is resolved when make completes successfully.

Written by the indexing model from the issue text.