Fix detection of newer MinGW toolchains in configure.exe

This fixes a regression in the new MinGW packages that are now built
with mkspec win32-g++-4.6: Since we failed to detect this as a MinGW
mkspec we missed to check for headers in all relevant paths, which
disabled windowsxp, windosvista styles by default.

This commit does not apply to Qt 5, similar issues there are being
fixed independently.

[ChangeLog][Windows] configure.exe now detects newer MinGW versions
correctly, enabling windowsxp and windowsvista styles by default.

Task-number: QTBUG-38706
Change-Id: Ib4e7af69e8040aaa28940f501cce5572588d4378
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Restore dirty line logic in RenderInline::destroy

In ea0cb7ad2066d9c3dc695ab2ebe724249b05ca0f the webkit version was
upgraded which effectively merged http://trac.webkit.org/changeset/85869
The follow up patch was never merged: http://trac.webkit.org/changeset/86060
associated with ​https://bugs.webkit.org/show_bug.cgi?id=60448

Change-Id: I7dff1733439bb3bcfecc767da9147aa09a3159cf
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>