Added the ability to escape the '$' character before '(' as "$$" which will allow using the "$()" and "$(())" expressions of bash, sh, and other shells.

[ivan2201]

I implemented the ability to escape the "$" character as "$$" if it appears before the "(" character.
More precisely, every "$$" sequence preceding the "(" character will be treated as an escaped '$' character.

I proposed this escaping method based on writing SHELL scripts in Makefiles.
This allows the use of SHELL "$(...)" or "$((...))" expressions within bjam expressions such as "actions { ... }" or "[ SHELL ... ]".

It's changing "$$(" interpreting, for example b2 rule:
rule test {
value = zero ;
echo $$$$(value) ;
}
Old execution result:
$$$zero
New execution result:
$$(value)

However, I did not find any use of the "$$(" sequence in the b2 files in the boost develop branch.

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[ivan2201]

It doesn't look like the pipeline error was related to my changes:

failure - unhandled exception {{{
b"PermissionError - [Errno 13] The process cannot access the file because it is being used by another process: 'C:\\\\Users\\\\VSSADM~1\\\\AppData\\\\Local\\\\Temp\\\\b2os2k7o_5'"
}}}
stacktrace {{{
b'at line 107 of D:\\a\\1\\s\\test\\test_all.py (run_tests)\n'
}}}

Is "Windows VS 2022 Test" pipeline can be restarted?

https://dev.azure.com/bfgroup/B2/_build/results?buildId=1544&view=logs&j=9d9f1044-8565-506c-f0ef-27b57a5df325&t=c8d70bf3-aa99-5445-a6f8-5caa8630d218&l=151