Some GNU C version guards implicitly include the clang compiler, because
clang reports itself as GCC 4.2.1.
This implicit inclusion doesn't happen if the guard requires a GCC
version above 4.2.1.
Fix two such guards to explicitly include clang where it does support
the guarded feature:
- curl/curl.h: use `typecheck-gcc.h` with clang.
llvm clang v14+ supports this. The corresponding Apple clang version
is also v14.
Ref: https://en.wikipedia.org/wiki/Xcode#Toolchain_versions
Apple clang v14 tested OK in CI:
https://github.com/curl/curl/actions/runs/16353901480/job/46207437204
- tool_urlglib: use `__builtin_mul_overflow()` with clang v8+.
llvm clang v3.8+ supports this, but to accommodate for Apple clang,
start with v8, the Apple version having the mainline v3.8 feature set.
Also fix compile warnings triggered by the above:
- lib1912: fix duplicate `;`:
```
tests/libtest/lib1912.c:44:57: error: empty expression statement has no effect; remove unnecessary ';' to silence this warning [-Werror,-Wextra-semi-stmt]
44 | print_err(o->name, "CURLOT_LONG or CURLOT_VALUES");
| ^
[...]
```
Ref: https://github.com/curl/curl/actions/runs/16351302841/job/46198524880?pr=17955#step:12:61
- lib2032: silence typcheck warning with a cast:
```
tests/libtest/lib2032.c:145:29: error: sizeof on pointer operation will return size of 'CURL **' (aka 'void **') instead of 'CURL *[3]' (aka 'void *[3]') [-Werror,-Wsizeof-array-decay]
145 | ntlm_easy + num_handles);
| ~~~~~~~~~ ^
```
Ref: https://github.com/curl/curl/actions/runs/16351302841/job/46198524880?pr=17955#step:12:86Closes#17955
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