9796dcacdc doc: Install only "-posix" MinGW compiler when possible (Hennadii Stepanov)
0bbae237a8 ci: Drop no longer needed `update-alternatives` (Hennadii Stepanov)
01d1845a80 build, qt: Specify QMAKE_CXX explicitly (Hennadii Stepanov)
Pull request description:
While changes introduced in bitcoin/bitcoin#22093 worked fine with Qt 5.12, after bumping Qt up to 5.15 the cross-compiling of `qt` package for Windows fails with `error: ‘mutex’ in namespace ‘std’ does not name a type`.
The first commit fixes this bug.
The second commit cleans up a related CI script.
The third commit improves related docs (see https://github.com/bitcoin/bitcoin/pull/22093#discussion_r680911586).
ACKs for top commit:
prusnak:
ACK 9796dca
Tree-SHA512: 0dc46c5dfab85bd6d2901052cd630e86f9b4e09c08ef87136b44ddecb1783cdf3cd0a6e67b95ac7a78da24cd7adedc88745f61f9a8d9993fbff26d33bf88d874