You can see how to add the USE_MEGA_8BIT_PORTC_SHIELD block to the special.h by looking at your existing USE_MEGA_8BIT_SHIELD and USE_MEGA_16BIT_SHIELD blocks
This is the important lines from my build report:
#warning USE_MEGA_8BIT_PORTC_SHIELD for Mihael54
^
In file included from C:\Users\David Prentice\Documents\Arduino\libraries\Mcufriend_kbv\MCUFRIEND_kbv.cpp:35:0:
C:\Users\David Prentice\Documents\Arduino\libraries\Mcufriend_kbv\utility/mcufriend_shield.h:18:2: warning: #warning WE ARE USING A SPECIAL CUSTOM DRIVER [-Wcpp]
#warning WE ARE USING A SPECIAL CUSTOM DRIVER
^
Using library SPI at version 1.0 in folder: C:\Program Files (x86)\Arduino-1.8.9\hardware\arduino\avr\libraries\SPI
Using library Adafruit_GFX_Library at version 1.7.3 in folder: C:\Users\David Prentice\Documents\Arduino\libraries\Adafruit_GFX_Library
Using library Mcufriend_kbv at version 2.9.9-Beta in folder: C:\Users\David Prentice\Documents\Arduino\libraries\Mcufriend_kbv
"C:\\Program Files (x86)\\Arduino-1.8.9\\hardware\\tools\\avr/bin/avr-size" -A "C:\\Users\\DAVIDP~1\\AppData\\Local\\Temp\\arduino_build_425669/graphictest_kbv.ino.elf"
Sketch uses 30538 bytes (12%) of program storage space. Maximum is 253952 bytes.
Global variables use 1600 bytes (19%) of dynamic memory, leaving 6592 bytes for local variables. Maximum is 8192 bytes.
Obviously this is untested. I don't have your hardware.
The #warning USE_MEGA_8BIT_PORTC_SHIELD for Mihael54 line is printed in Orange
You only get the warning when you build for the first time. Subsequent builds just say:
Using previously compiled file:
David.