__has_include is part of C++17, but moc did not handle it so far.
This commit fixes moc to correctly support it.
It should be noted that support for __has_include relies on all
necessary include paths being passed to moc.
Pick-to: 6.10 6.9 6.8
Fixes: QTBUG-136097
Change-Id: I7284e97dea12d1637b38349d32e090c0102124e7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
f2ef861688