Escape CBMC makefile help path separators

[Old-Ding]

Description

The CBMC makefile generator help text includes a Windows path example with single backslashes inside a normal Python string literal. Python reports this as an invalid escape sequence SyntaxWarning.

This escapes the backslashes in the source string while keeping the rendered help text unchanged.

Test Steps

• python -W error::SyntaxWarning -m py_compile test/cbmc/proofs/make_proof_makefiles.py
• python test/cbmc/proofs/make_proof_makefiles.py --help and verified the H_INC = /Imy\cool\directory help output is unchanged.
• Repository Python warning scan with SyntaxWarning promoted to errors reports TOTAL 0.
• git diff --check -- test/cbmc/proofs/make_proof_makefiles.py
• git ls-files --eol test/cbmc/proofs/make_proof_makefiles.py

Duplicate check:

• No open PR or issue found for make_proof_makefiles, SyntaxWarning, or invalid escape.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

N/A

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