linux-icc: always compile applications as position-independent execs

The problem isn't the compiler, but the linker, so we ought to turn this
flag on for all ELF-based systems where we compile Qt with -Bsymbolic,
but so far only the Intel compiler has been affected. We can turn it on
as needed for other systems.

The cause of the problem is the way that ICC materializes the pointer-
to-member-function: it stores the full 2*sizeof(void*) data in an
anonymous variable, so that it can load it in one go, such as one 16-
byte SSE aligned load on 64-bit systems. That relocation in a data
variable gets turned into a fixed-position copy relocation by the
linker, which breaks the signal-identification mechanism.

GCC and Clang are likely to be affected if anyone did:

    static const auto destroyed = &QObject::destroyed;
    QObject::connect(obj, destroyed, [](){});

Task-number: QTBUG-52439
Change-Id: I0d69eaf61af149db9574fffd149d0cfb08459c33
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

mkspecs/linux-icc/qmake.conf

@@ -79,6 +79,7 @@ QMAKE_LINK              = icpc
 QMAKE_LINK_SHLIB        = icpc
 QMAKE_LFLAGS            =
 QMAKE_LFLAGS_RELEASE    =
+QMAKE_LFLAGS_APP        = -pie
 QMAKE_LFLAGS_DEBUG      =
 QMAKE_LFLAGS_SHLIB      = -shared -shared-intel
 QMAKE_LFLAGS_PLUGIN     = $$QMAKE_LFLAGS_SHLIB