When compiling Doxygen 1.7.4 with GCC v 6.4.0 for HP Itanium 11i3 I found I had to modify the following file:

./qtools/qthread_unix.cpp

to include a system header file to enable perror to be defined:

# line 46, insert
include <stdio.h>


If I didn't do this then the following is reported by g++ after gmake executes the following line:

g++ -c -DQT_LITE_UNICODE -D_LARGEFILE_SOURCE -Wall -W -I. -o ../objects/qthread_unix.o qthread_unix.cpp
qthread_unix.cpp: In static member function 'static int QThread::idealThreadCount()':
qthread_unix.cpp:190:32: error: 'perror' was not declared in this scope
gmake[2]: *** [../objects/qthread_unix.o] Error 1

Fortunately I found perror in the man pages ;o)