Do not use -Wl,--as-needed with emscripten.

On some platform, fmt is using the following link flag:
```cmake
  target_link_libraries(fmt -Wl,--as-needed)
```

This is not supported by wasm-ld as of today.
This patch stop using it when compiling using emscripten.

Bug: https://github.com/fmtlib/fmt/issues/1736

CMakeLists.txt

@@ -223,7 +223,7 @@ if (CMAKE_BUILD_TYPE STREQUAL "Debug")
 endif ()
 
 if (BUILD_SHARED_LIBS)
-  if (UNIX AND NOT APPLE AND NOT ${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
+  if (UNIX AND NOT APPLE AND NOT ${CMAKE_SYSTEM_NAME} MATCHES "SunOS" AND NOT EMSCRIPTEN)
     # Fix rpmlint warning:
     # unused-direct-shlib-dependency /usr/lib/libformat.so.1.1.0 /lib/libm.so.6.
     target_link_libraries(fmt -Wl,--as-needed)