[backport#15457] Check std::system for -[alert|block|wallet]notify

Summary:
f874e14cd3c84cd412bd3fb42b3ee1706ca6a267 [build]: check std::system for -[alert|block|wallet]notify (Sjors Provoost)
cc3ad56ff2bc2583fe68c4a9e0b41072a47c0b07 [build] MSVC: set HAVE_SYSTEM for desktop apps (Sjors Provoost)
c1c91bb78d7267f01ee3a3c156c218b46a92cd39 [build] detect std::system or ::wsystem (Sjors Provoost)

Pull request description:

Platforms such as iOs and Universal Windows Platform do not support launching a process through system().

https://github.com/bitcoin/bitcoin/pull/15457/files

Backport of Core PR15457

Test Plan:

ninja check

don't have a windows machine atm to test this on

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7031