[build] Use a submodule for mapbox-gl-js rather than an npm dependency

[jfirebaugh]

Refs #5249

[mention-bot]

@jfirebaugh, thanks for your PR! By analyzing this pull request, we identified @tmpsantos, @incanus and @mourner to be potential reviewers.

[jfirebaugh]

See in particular #5249 (comment) for the rationale here.

@kkaefer @tmpsantos @1ec5 Any reservations about testing this out for a few days?

[1ec5]

I think it should be OK. We’re still using npm on the release-ios-v3.4.0 branch, which is going to be active for the next few weeks, with periodic merges back to master. It just means we have to be careful not to accidentally bring a submodule change along for the ride as we perform merges.

[tmpsantos]

Thanks a lot for that!

[tmpsantos]

Any reservations about testing this out for a few days?

No problem at all. Regarding the CI caching issue we could have a checkout of the repo on the travis cache directory and copy it to the submodule location on the travis shell script.

[kkaefer]

Can we try using externalproject_add with the GIT_SUBMODULES setting instead?

[jfirebaugh]

I wasn't able to get this to work. It looks to me like the GIT_SUBMODULES option is for initializing submodules of the external project, rather than using a submodule as the method to obtain the external project itself.

[tmpsantos]

I've been playing with externalproject_add for a while on the CMake build I'm maintaining for building Mapbox GL Native 100% offline and for Qt + Windows. On the build file for predownloading all of our dependencies I'm doing something like:

include(ExternalProject)

# Macro for cloning and downloading our header-only dependencies
macro(add_dep package tag git)
    externalproject_add(${package}
        URL https://github.com/${git}/archive/${tag}.zip
        CONFIGURE_COMMAND ""
        BUILD_COMMAND ""
        INSTALL_COMMAND ""
    )

    add_dependencies(mbgl-core ${package})

    externalproject_get_property(${package} SOURCE_DIR)

    target_include_directories(mbgl-core PRIVATE ${SOURCE_DIR}/include)
    target_include_directories(qmapboxgl PRIVATE ${SOURCE_DIR}/include)

    set(${package}_SOURCE_DIR ${SOURCE_DIR})
endmacro()

Two things: It is a lot faster to get the tarball rather then cloning the repo and CMake will try to configure + build + install, but you can override it.