Fix configure & qmake compilation with a future MSVC version

When MSVC supports ref-qualified members, we need to ensure that
qstring_compat.cpp can see the non-qualified definitions in qstring.h,
which means no precompiled header.

Alternatively, for a bootstrapped build we could not compile
qstring_compat.cpp or #ifndef the functions.

Change-Id: I8ece34503060f0b4b0f8f2df2fb9b0fb1311e269
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Add other integer overloads to QFlag's constructor

This avoids "change of sign" warnings as found by ICC when the high
bit is set. This often happens when you do
    X & ~Y

as ~Y probably has bit 31 on. If the enum is unsigned, then there's a sign conversion.

Change-Id: Ia5f221d928ac0155f4504a70c4046e60c25fbf3b
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>