AIX wants string.h in signal.c (and is right to do so) :
gcc -Iinclude -Wall -O2 -g -DTPROXY -DENABLE_POLL -DCONFIG_HAPROXY_VERSION=\"1.3.18\" -DCONFIG_HAPROXY_DATE=\"2009/05/10\" -c -o src/signal.o src/signal.c
src/signal.c: In function 'signal_init':
src/signal.c:32: warning: implicit declaration of function 'memset'
src/signal.c:32: warning: incompatible implicit declaration of built-in function 'memset'
These functions will be used to deliver asynchronous signals in order
to make the signal handling functions more robust. The goal is to keep
the same interface to signal handlers.