configure: Change default of CFG_REDUCE_RELOCATIONS to "no".

Suppress QMAKE_LFLAGS_BSYMBOLIC_FUNC (-Bsymbolic-functions for g++)
by default since it causes crashes with gcc 5.X. Since applications
compiled with gcc 5.X might run against Qt compiled with gcc 4.9.X
and CLANG might also be affected, turn it off by default.

Task-number: QTBUG-45755
Change-Id: I5704c3156db6b6f74e1c14576e5d02bcbd3082a4
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: BogDan Vatra <bogdan@kde.org>