dynamically generate curl header file with macro if curl installed

[lalitb]

Based on comment here:
#445 (comment)
Instead of defining macro for all the cpp source files which use curl based http client, it's suggested to instead automatically generate/update the header file for WITH_CURL macro, and ensure we include this header file wherever needed.

[jsuereth]

Note: There's also an aspect of the BAZEL build where it'd be ideal if we had a header file that included the necessary -D. See #444 for an example in bazel where libraries using CURL need to add the defines.

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[github-actions]

This issue was marked as stale due to lack of activity. It will be closed in 7 days if no furthur activity occurs.

[github-actions]

This issue was marked as stale due to lack of activity. It will be closed in 7 days if no furthur activity occurs.