Today while trying to figure out a strange build error with a student (mozconfig doesn't like CR as much as CRLF), Ted decided to share a bit about the inner life of mozconfig. I repeat it with permission for your reading pleasure.
Act I
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You run "make -f client.mk"
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The run_for_side_effects line runs [mozconfig2client-mk](see http://mxr.mozilla.org/mozilla-central/source/build/autoconf/mozconfig2client-mk)
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This uses mozconfig-find to find the mozconfig to use
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Your mozconfig is sourced as a shell script
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In mozconfig2client-mk, "mk_add_options" is defined as a shell function that does some escaping and echoes whatever is listed after it to a temp file (ac_add_options in that file is defined as a no-op)
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client.mk then includes that temp file, so anything in it is treated as makefile source
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mk_add_options FOO=bar just winds up as the makefile assignment "FOO=bar"
Act II -
configure repeats this dance, but uses mozconfig2configure instead
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this does the reverse: ac_add_options does some quoting and echoes the result, mk_add_options is a no-op
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finally, configure sources the result as a shell script (with some hackery in there)
[end.]