qt: Makefile.am: remove non-POSIX make constructs

$(filter), $(filter-out) and $(notdir) cannot be used in automake since they won't be replaced by POSIX-make code.

This commit replaces the usage by explicitely setting the correct values at the correct location.

@fuzun assigned as I don't really know the underlying task it was doing before, and I didn't included the changes from !5235 (merged) so it's not fixing anything regarding it. I've checked both with --disable-qt-qml-cache and --enable-qt-qml-cache but with an heavily modified buildsystem since the current state wasn't compiling at all for me.