These flags are only needed for builds involving shared libraries,
and can cause undesirable behavior when they are not needed.
They also have variants with values like `=2`. For now, just require
projects to add the flags when needed so they have full control.
In the future we may be able to add an abstraction to control them,
and automatically enable them when linking to shared libraries
or creating them.
Fixes: #27232
Issue: #27240
Place the flag early on executable link lines so that projects can
override it, e.g., with `-sMAIN_MODULE=2` or `-sMAIN_MODULE=0` via
`target_link_options`.
The `-sSIDE_MODULE` flag already appears early enough on shared library
link lines.
Issue: #27232
Emscripten provides a combined toolchain file and platform module that
predates CMake upstream support. It disables support for shared libs.
Fix commit 96d9b94a98 (Emscripten: Add platform modules, 2025-05-16) to
avoid linking with `-sMAIN_MODULE` or `-sSIDE_MODULE` flags, meant to
support shared libraries, when using the emsdk toolchain file.
Fixes: #27232
Update commit 96d9b94a98 (Emscripten: Add platform modules, 2025-05-16)
to account for commit 67639002ad (Ninja,Makefile: Drop now-unused
placeholders from link rule variables, 2025-06-09, v4.1.0-rc1~37^2~1).
Brad King