qmake: updates to the parsing code of external properties

Change-Id: I7555de5c72a9250b31e20fc60e39680d19882fcb
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from qtcreator/2cb7c81e620d224d386860a637dc889acb15435e)
(cherry picked from qtcreator/89868ee2b9093ecf40602ae302b991d6a60014b0)
(cherry picked from qtcreator/03e699ce2985eedcd33d247aa47d04b14bc4bc04)
(cherry picked from qtcreator/61419e7bf0f3bff6dcf63876b05b72c56e60c2a8)
(cherry picked from qtcreator/19eaf87ef95a510351557119a955223a4aeea7b3)
(cherry picked from qtcreator/3080bda0661989e88dfa62101b4c3f5d5e6754a1)
(cherry picked from qtcreator/99714239b616e628ff4e0afe3db7eb7511ccf569)
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>