- appveyor: add build-only job for clang-cl.
- cmake: `-pedantic-errors` enables `-Werror,-Wlanguage-extension-token`
automatically, which makes `__int64` detection fail.
Explictly disable this compiler warning for clang-cl to make the
feature detection work and to accept `__int64` in the source code.
- cmake: disable `-Wlanguage-extension-token` warning for clang-cl
to fix these when encountering `__int64`:
```
lib/formdata.c(797,29): error : extension used [-Werror,-Wlanguage-extension-token]
lib/warnless.c(117,33): error : extension used [-Werror,-Wlanguage-extension-token]
lib/warnless.c(60,28): message : expanded from macro 'CURL_MASK_SCOFFT'
lib/warnless.c(59,38): message : expanded from macro 'CURL_MASK_UCOFFT'
include\curl/system.h(352,40): message : expanded from macro 'CURL_TYPEOF_CURL_OFF_T'
```
- make `__GNUC__` warning suppressions apply to `__clang__` too.
Necessary for clang-cl, which defines the latter, but not the former.
(Regular clang defines both.)
- examples: fix clang-cl compiler warning in `http2-upload.c`.
```
docs\examples\http2-upload.c(56,5): error : no previous prototype for function 'my_gettimeofday' [-Werror,-Wmissing-prototypes]
docs\examples\http2-upload.c(56,1): message : declare 'static' if the function is not intended to be used outside of this translation unit
```
- unit2604: add missing `#pragma GCC diagnostic pop`.
Follow-up to e53523fef0#14859
- unit1652: limit compiler warning suppression to GCC.
They do not affect clang builds.
Follow-up to 71cf0d1fca#14772Closes#15449
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