When using MinGW compiler and Qt is configured with "-static-runtime",
we should pass "-static" to g++ while linking, like Qt 5, instead of
"-Wl,-Bstatic", to get rid of dependencies on libgcc_s_seh-1.dll,
libwinpthread-1.dll and libstdc++-6.dll.
Because syncqt doesn't link to any Qt library,
"target_link_options(${target} INTERFACE -static)" has no effect on it.
So we should use "PRIVATE" instead of "INTERFACE" for executables.
Pick-to: 6.6 6.5
Change-Id: Icf551783f92ef3615b3840c9af16d163eee09fdb
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
9256d9e7b6