The CMake documentation states that the AUTOMOC_MACRO_NAMES property of
a target is initially set to CMAKE_AUTOMOC_MACRO_NAMES, so do not
override it in qt_enable_autogen_tool.
Instead, append the extra macro names not set by the upstream CMake
CMAKE_AUTOMOC_MACRO_NAMES variable.
Add a test for this.
Change-Id: Ib0ef28e7fc9c5f9559150bbe73ffdeac767adc82
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
