The fix for SV 10593 caused recursive expansion errors when exporting
a variable that contains a $(shell ...) invocation. If we see this
type of recursion, ignore it and expand to the empty string rather
than failing.
* src/variable.h (env_recursion): New global variable.
* src/variable.c (target_environment): If creating the environment
for a $(shell ...) function increment env_recursion. Remove the
check for expansion in a shell function context.
* src/expand.c (recursively_expand_for_file): Check for recursive
expansion in a $(shell ...) environment context and if present,
show the verbose message and return the empty string.
* tests/scripts/functions/shell: Add a test for this situation.
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