Even though `PICKY_COMPILER=ON` is the default, warnings were not
enabled when using llvm/clang, because `CMAKE_COMPILER_IS_CLANG` was
always false (in my tests at least).
This is the single use of this variable in curl, and in a different
place we already use `CMAKE_C_COMPILER_ID MATCHES "Clang"`, which works
as expected, so change the condition to use that instead.
Also fix the warnings uncovered by the above:
- lib: add casts to silence clang warnings
- schannel: add casts to silence clang warnings in ALPN code
Assuming the code is correct, solve the warnings with a cast.
This particular build case isn't CI tested.
There is a chance the warning is relevant for some platforms, perhaps
Windows 32-bit ARM7.
Closes#9783
811c799f2d