Public curl headers are best not to define 3rd-party or system macros.
Introduce `CURL_HAS_DECLSPEC_ATTRIBUTE` to cover this system macro and
use it.
Detected by test1167 after dropping the indentation:
```
test 1167...[Verify curl prefix of public symbols in header files]
/usr/bin/perl -I. -I. returned 1, when expecting 0
1167: exit FAILED
== Contents of files in the log/10/ dir after test 1167
=== Start of file server.cmd
Testnum 1167
=== End of file server.cmd
=== Start of file stdout1167
Bad symbols in public header files:
__has_declspec_attribute(x)
=== End of file stdout1167
```
Ref: https://github.com/curl/curl/actions/runs/13533200900/job/37819784405?pr=16490#step:42:2087
Ref: https://clang.llvm.org/docs/LanguageExtensions.html#has-declspec-attribute
Follow-up to 50482b8c0a#3616
Ref: #16496 (fixing test1167)
Closes#16491
You find a mix of various documentation in this directory and subdirectories,
using several different formats. Some of them are not ideal for reading
directly in your browser.
If you would rather see the rendered version of the documentation, check out the
curl website's documentation section for
general curl stuff or the libcurl section for
libcurl related documentation.
dbbbf717f3
