Failed to build from source under GCC 6.1.x

[hosiet]

See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129 and https://bugreports.qt.io/browse/QTBUG-53367.

...
g++ -c -pipe -g -O2 -fdebug-prefix-map=/<>=. -fPIE -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -std=gnu++0x -Wall -W -D_REENTRANT -fPIC -DLIBQTELEGRAM_LIBRARY -DQT_NO_DEBUG -DQT_MULTIMEDIA_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -I. -isystem /usr/include -isystem /usr/local/include -isystem /usr/include/i386-linux-gnu/qt5 -isystem /usr/include/i386-linux-gnu/qt5/QtMultimedia -isystem /usr/include/i386-linux-gnu/qt5/QtGui -isystem /usr/include/i386-linux-gnu/qt5/QtNetwork -isystem /usr/include/i386-linux-gnu/qt5/QtCore -I. -I/usr/lib/i386-linux-gnu/qt5/mkspecs/linux-g++ -o telegramfunctionobject.o telegram/functions/telegramfunctionobject.cpp
In file included from /usr/include/c++/6/bits/stl_algo.h:59:0,
                 from /usr/include/c++/6/algorithm:62,
                 from /usr/include/i386-linux-gnu/qt5/QtCore/qglobal.h:88,
                 from /usr/include/i386-linux-gnu/qt5/QtCore/QtGlobal:1,
                 from telegram/functions/telegramfunctionobject.h:8,
                 from telegram/functions/telegramfunctionobject.cpp:5:
/usr/include/c++/6/cstdlib:75:25: fatal error: stdlib.h: No such file or directory
 #include_next 
                         ^
compilation terminated.
Makefile:5205: recipe for target 'telegramfunctionobject.o' failed

Looks like some tweaks in building system is needed.

[farseerfc]

This should be fixed in Qt's side. The issue is affacting all Qt project built with qmake.
For now, calling qmake with QMAKE_CFLAGS_ISYSTEM= (that is empty value) is a workaround to address this issue.