Clang / musl libc: workaround typedef redefinition
id_t is already defined in bits/alltypes.h, adds __DEFINED_id_t check
Fixes the following error reported at https://bugs.gentoo.org/894510
libfakeroot.c:147:13: error: typedef redefinition with different types ('int' vs 'unsigned int')
typedef int id_t;
^
/usr/include/bits/alltypes.h:240:18: note: previous definition is here
typedef unsigned id_t;
^