qmake: Expose ARGC to qmake functions to be able to check argument count

ARGS already exists, but is a flattened list of the arguments, so both
foo(bar, baz) and foo(bar baz) will give count(ARGS, 2), making it
unreliable for validating arguments to qmake functions.

Change-Id: I0bcc16614c64000169431327da48fd1a26708e67
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/09205d573413da8a2ac3826198fe36bb2dc4349f)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>