Alloca() is not supported on all platforms, like
INTEGRITY on ARM, so adding a configure check for it.
This can be used when building QtQml and 3rd party
code, in particular PCRE2 and SQLite.
Change-Id: I9785e16c21f67d1a68fef567e18c3356170f027e
Reviewed-by: Lars Knoll <lars.knoll@qt.io>