[Interest] building Qt [4.8.7 with gcc 5] and link-time optimisation

[René J. V. Bertin]

Concerning my test Qt5 build with LTO:

How well supported/tested is doing that in out-of-source "shadow build"? After 
using -march=core2 instead of -march=native (and thus avoiding the use of SSE4 
instructions), I'm running into another issue when I execute qmake -config ltcg 
immediately after running configure:

qt-everywhere-opensource-src-5.4.2/qtbase/src/network/access/qftp.cpp:42:10: 
fatal error: 
      'qcoreapplication.h' file not found
#include "qcoreapplication.h"
         ^
1 error generated.


I have a hunch this may be related to the fact that I'm running configure and 
build in a virgin build directory but with Qt5 already installed as configured 
(i.e. I'm rebuilding with ltcg).
I do not get that error when I do NOT execute qmake -config ltcg after 
configure, and also do not get it when I first build without ltcg and then call 
qmake -r config ltcg in the "finished" build directory. I'm using bin/qmake from 
the build directory itself, btw.

I'll be testing later if there's any difference when I edit the makefile 
generated by configure, so that it calls $(QMAKE) -config ltcg instead of simply 
$(QMAKE).

R.