Allow defining zopengl export-linkage at build time

[pascalPost]

This change allows zopengl export-linkage to be selected at build time:

• internal for Emscripten builds
• strong for desktop builds

Using strong linkage with Emscripten can break builds due to symbol collisions with Emscripten-exported symbols, so Emscripten now uses internal linkage by default.

Example usage (build.zig):

const zopengl_export_linkage: std.builtin.GlobalLinkage =
    if (target.result.os.tag == .emscripten) .internal else .strong;

const zopengl =
    b.dependency("zopengl", .{
        .target = target,
        .export_linkage = zopengl_export_linkage,
    });

[hazeycode]

Thanks!