System LD_LIBRARY_PATH omitted when building qttranslations

When building (e.g.) qttranslations the system LD_LIBRARY_PATH is omitted.
One example scenario where this causes issues is when Qt5 is linking against custom built ICU libraries.

qttools/bin/lrelease: error while loading shared libraries: libicui18n.so.49: cannot open shared object file: No such file or directory
make[1]: *** [assistant_cs.qm] Error 127
make[1]: Leaving directory `<build_dir>/qttranslations/translations'
make: *** [sub-translations-make_first] Error 2

Open e.g. /qttranslations/translations/Makefile

ts-qt-zh_CN:
cd /src && QT_PLUGIN_PATH=<build_dir>/qtbase/plugins LD_LIBRARY_PATH=<build_dir>/qtbase/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH} QMAKE=<build_dir>/qtbase/bin/qmake <build_dir>/qttools/bin/lupdate -locations relative -no-ui-lines -I../include -I../include/Qt 3rdparty/phonon 3rdparty/webkit activeqt corelib declarative gui multimedia network opengl plugins qt3support script scripttools sql svg xml xmlpatterns -ts <build_dir>/qttranslations/translations/qt_zh_CN.ts

-> put printenv command first and check the LD_LIBRARY_PATH value from the output. You can find that the system LD_LIBRARY_PATH value is missing.