pam does not build on Th/Ti

Compilation stops at pam_console:

handlers.c: In function 'call_exec':
handlers.c:199: warning: cast discards qualifiers from pointer target type
handlers.c: In function 'execute_handler':
handlers.c:213: error: 'sighandler_t' undeclared (first use in this function)
handlers.c:213: error: (Each undeclared identifier is reported only once
handlers.c:213: error: for each function it appears in.)

_GNU_SOURCE is defined but __USE_GNU is not and therefore sighandler_t is not defined in signal.h