QCoreTextFontDatabase: Fix font weight value when populating a family

kCTFontWeightTrait returns a normalized value between -1.0 (lightest)
and 1.0 (heaviest), 0.0 being the regular font weight. The threshold
values used in this change have been estimated from the weight values
of fonts from the Helvetica Neue and Myriad Pro font families.

Change-Id: I49de8e8bd5894107de4842aeda7ace2e83f95be3
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Work around ICC bug in local static symbols for Q_GLOBAL_STATIC

When compiling the innerFunction() of the Q_GLOBAL_STATIC expansion, ICC
emits global symbols for the internal "holder" local static variable and
its guard. If there are two global statics of the same name in two
different .cpp files, the linker will incorrectly merge the two "holder"
variables.

This was noted between the "customTypes" global statics of qmetatype.cpp
and qdbusmetatype.cpp in a static build.

The C++ standard requires that local static variables declared in inline
functions must be the same, regardless of whether the body of the
function got inlined or not. The IA-64 C++ ABI does that by requiring
local static symbols for inline functions to be global and mergeable
("link once"). However, two functions in anonymous namespaces in
different files are not considered to be the same function, so their
local statics should not be merged. This is where ICC failed: the local
statics are global and mergeable, even though the function is in an
anonymous namespace. ICC correctly emits the function itself as a local
symbol.

Alternative solutions were:
  1) add "static", but you can't use a static symbol in a template
     parameter in C++98 mode
  2) remove the "inline" keyword, but then GCC 4.8 will not inline

Intel issue ID: 6000058488
Task-number: QTBUG-40053
Change-Id: I307622222499682dde711b2771c8cf7557400799
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>