INITIAL_MEMORY and PTHREAD_POOL_SIZE are linker flags only
USE_PTHREADS is both linker and compiler arguments.
Also increase default INITIAL_MEMORY
Pick-to: 6.3
Change-Id: Id1998efbf1d6de901f404db7e988f6cafd547a39
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>