automoc: Read target defines unconditionally

Commit 429e3699 (Process COMPILE_DEFINITIONS as generator expressions
in QtAutomoc., 2013-01-25) introduced a check for the existence of the
COMPILE_DEFINITIONS property on the target before processing it.

At the time, compile definitions from linked targets were added to the
COMPILE_DEFINITIONS property by target_link_libraries. The way such
transitive compile definitions worked was changed in subsequent
commit a1c4905f (Use the link information as a source of compile
definitions and includes., 2013-02-12), so that the COMPILE_DEFINITIONS
property may be empty, despite the fact that there are definitions
which should be used.
Merge topic 'fix-moc-with-empty-COMPILE_DEFINITIONS'

33ed186 automoc: Read target defines unconditionally