> Hello,
>
> I found that tqt.h is modified only at install stage.

That is correct, however the automake/autoconf system defines USE_QT3 or
USE_QT4 based on the Qt version specified in admin/acinclude.m4.in.

The new CMake-based build system will also need to define USE_QT3 or
USE_QT4 when it builds any of the Trinity modules.  This is to allow
conditional code within Trinity so as to maintain build compatibility with
both versions of Qt.

> If I define USE_QT3
> before compilation, the compilation stop with this error:
>
> tqinputcontext.h:32:27: error: qinputcontext.h: No such file or directory
>
> This is intended behaviour? If yes, for what purpose is designed in this
> way?

No, that is not the intended behaviour.  Do you have the qinputcontext.h
header file present on your system?  The old autoconf/automake system
actively searched for the Qt headers and added the that directory to the
include search path in the gcc flags.

Also please be aware that for all modules that rely on TQt, you will need
to detect the presence of the TQt headers, and pass the "-include tqt.h"
flag to gcc.  You should also add the TQt header directory to gcc's
include search path.

Hope this helps some!

Tim