During preprocessing, we find directives, and if they specify including
other source files, we remove the directive, allocate a new block of
memory, copy the first block, append the new source file, and append the
second block.
Previously, the value of the source pointer was passed into the
preprocessing function. When the memory would be (re)allocated, the
pointer value could change, but the original pointer would not be
updated.
Now, the pointer is passed by address, and updated when the memory it
points to changes.
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