Overriding -jsD directives causes an error

[juj]

In native C compiler, one can override or repeat preprocessing directives on the command line, e.g.

$ emcc ... -DFOO=1 -DFOO=1

The -jsD setting was added to specify preprocessing directives in JS library code. However, attempting to override or repeat such directives currently causes an error:

$ emcc test/hello_world.c -jsDFOO=1 -jsDFOO=1
emcc: error: -jsDFOO=1: cannot change built-in settings values with a -jsD directive. Pass -sFOO=1 instead!

where the error is unrelated/confused with -sFOO settings.

[sbc100]

I guess its just assuming if setting already exists then it is a built-in one.

I think this is very rarely used feature so its probably not a high priority fix but maybe and easy one of someone want the take it on ( The tests live in test_jslib_custom_settings in test/test_jslib.py).