Avoid changing working directory EMCC_BATCH_BUILD is unset. NFC

[sbc100]

When not using EMCC_BATCH_BUILD the working directory does not matter.

[kripken]

When not using EMCC_BATCH_BUILD the working directory does not matter.

This is a large file and after skimming relevant parts I'm not sure how to convince myself that is the case?

[sbc100]

When not using EMCC_BATCH_BUILD the working directory does not matter.

This is a large file and after skimming relevant parts I'm not sure how to convince myself that is the case?

Firstly, this change only effects users who do EMCC_BATCH_BUILD=0.. which is just me occasionally i think.

The default use case EMCC_BATCH_BUILD=1 is unchanged.

The reason EMCC_BATCH_BUILD=1 needs to control the CWD is that it uses relative paths to refer to the source files:

See:

# Use relative paths to reduce the length of the command line.                               
# This allows to avoid switching to a response file as often.                                
src = os.path.relpath(src, build_dir)

This only happens for the batch builder, and its one of the reasons I don't like the batch builder: Because all the errors like ../../../system/libc/myfile.c. This means that filenames in the error message are less useful. With the non-batch builder all the filenames are absolute.

[sbc100]

I refactored and added some comments.