I'm confused. What is 0x18AD? The ELF package has a bunch of constants define for use in the e_machine field of the elf32 and elf64 headers. These are simple integers ranging from 0 (EM_NONE) to 247 (EM_BPF). These are set by whatever application creates the ELF formatted file. To my knowledge 0x18AD doesn't fit this definition!

There is a comment in the elf.h file suggesting using large random numbers for new architectures. But there is a definition for the 32 bit AVR architecture. Perhaps 0x18AD is left over from ancient history when the 32 bit AVR architecture was new? If that's the case then there is a need for two constants - the official value (185) and the unofficial value (0x18AD).

Compiling version 6.3 from source on Ubuntu 18.04 reports a redefinition warning that seems to be currently benign but may cause problems in the future. The error is

gcc -DHAVE_CONFIG_H -I. -I..  -DCONFIG_DIR=\"/home/braun/loca/etc\"  -Wall -Wno-pointer-sign -g -O2 -MT libavrdude_a-fileio.o -MD -MP -MF .deps/libavrdude_a-fileio.Tpo -c -o libavrdude_a-fileio.o `test -f 'fileio.c' || echo '../'`fileio.c
../fileio.c:37:0: warning: "EM_AVR32" redefined
 #define EM_AVR32 0x18ad         /* inofficial */
 
In file included from /usr/include/libelf.h:36:0,
                 from ../fileio.c:33:
/usr/include/elf.h:323:0: note: this is the location of the previous definition
 #define EM_AVR32 185 /* Amtel 32-bit microprocessor */
 
At least a comment should be added explaining why the redefinition is needed. If the intent was to actually redefine the constant a "#undef" should precede the redefinition.

A second warning occurs stating that misleading indentation is present. The warning is

gcc -DHAVE_CONFIG_H -I. -I..  -DCONFIG_DIR=\"/home/braun/local/etc\"  -Wall -Wno-pointer-sign -g -O2 -MT libavrdude
_a-avr.o -MD -MP -MF .deps/libavrdude_a-avr.Tpo -c -o libavrdude_a-avr.o `test -f 'avr.c' || echo '../'`avr.c
../avr.c: In function ‘avr_tpi_chip_erase’:
../avr.c:83:5: warning: this ‘while’ clause does not guard... [-Wmisleading-indentation]
     while (avr_tpi_poll_nvmbsy(pgm));
     ^~~~~
../avr.c:85:3: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘while
’
   err = pgm->cmd_tpi(pgm, cmd, sizeof(cmd), NULL, 0);
   ^~~

BTW - I created a ./build directory in which I performed the build steps.