More precisely, free us from specific Q_MOC_RUN/QT_NO_META_MACROS code
paths in QtCore/qobjectdefs.h.

In order to track signals and slots we provided custom definitions of
e.g. "Q_SIGNAL" by including qt5-qobjectdefs-injected.h with "-include".
We also had to ensure that those macros were not overwritten by Qt's
qobjectdefs.h, which we did by defining QT_NO_META_MACROS. However, this
came with a cost: we needed to replicate all the other macro definitions
in the Q_MOC_RUN/QT_NO_META_MACROS code path, e.g. Q_INTERFACES. This
bound us to specific versions of qobjectdefs.h and occasionally we had
to adapt (see change 4eafa2e0).

The new approach wraps Qt's qobjectdefs.h with the help of
"include_next". In the wrapper header, we only redefine what is
necessary.

The "include_next" directive is originally a GNU extension. Clang seems
to support it unconditionally, as [1] implicitly states.

[1] http://clang.llvm.org/docs/LanguageExtensions.html#include-file-checking-macros



Change-Id: Ic1a263f94b178349cb32bfdbb074ad5e6e0761ee
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>