Building with -sMAIN_MODULE vs with -sSTRICT and -l<foo> linkage

When building with `-sSTRICT`, one must explicitly link in e.g. GL and HTML5 JS libraries with `-lGL` and `-lhtml5`. The intent was to avoid accidental pollution of symbols into users's builds if they so need.

When building with `-sMAIN_MODULE` however, it looks like one cannot manually pass `-lGL` or `-lhtml5`, or errors about duplicate symbols will result.

Seen while fixing PR https://github.com/emscripten-core/emscripten/pull/24431#discussion_r2114738526