Add support for building sdl2-client installer to meson-installer_build.sh
Builds on #44675. Will include gtk3 modpack installer initially, to use existing gtk3 library installation implementation. Adding gtk4 library installation and switching to gtk4 modpack installer left to a future ticket.
meson-winbuild.sh already builds gtk4 modpack installer as part of sdl2 build. It would have been counter-productive to change that back to gtk3 one, so did the meson-installer_build.sh change directly to gtk4 modpack installer using one.
Add support for building sdl2-client installer to meson-installer_build.sh
Builds on #44675. Will include gtk3 modpack installer initially, to use existing gtk3 library installation implementation. Adding gtk4 library installation and switching to gtk4 modpack installer left to a future ticket.