This is the compiler's report:

/opt/local/bin/gcc-apple-4.2 -DHAVE_CONFIG_H -I. -I.. -DLOCALEDIR=\"/opt/local/share/locale\" -DGNUPG_BINDIR="\"/opt/local/bin\"" -DGNUPG_LIBEXECDIR="\"/opt/local/libexec\"" -DGNUPG_LIBDIR="\"/opt/local/lib/gnupg\"" -DGNUPG_DATADIR="\"/opt/local/share/gnupg\"" -DGNUPG_SYSCONFDIR="\"/opt/local/etc/gnupg\"" -DGNUPG_LOCALSTATEDIR="\"/opt/local/var\"" -DGNUPG_DEFAULT_PINENTRY="\"/opt/local/bin/pinentry\"" -I/opt/local/include -I/opt/local/include -I/opt/local/include -DKEYBOX_WITH_X509=1 -I/opt/local/include -I/opt/local/include -Wall -Wno-pointer-sign -Wpointer-arith -pipe -Os -arch ppc -MT keyboxd-backend-support.o -MD -MP -MF .deps/keyboxd-backend-support.Tpo -c -o keyboxd-backend-support.o test -f 'backend-support.c' || echo './'backend-support.c
In file included from keybox-defs.h:37,

from backend-support.c:32:

keybox.h:36: error: redefinition of typedef 'KEYBOX_HANDLE'
backend.h:28: error: previous declaration of 'KEYBOX_HANDLE' was here
make[2]: *** [keyboxd-backend-support.o] Error 1

This Mac is PowerPC based. On an intel based Mac with an almost modern macOS the then used Clang compiler has no problem with the two typedef's. The same is true for GCC 7 on this old PPC Mac with "Tiger".

A bit more can be found in the external link