cmake: really enable warnings with clang

[vszakats]

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.

  There is a chance the warning is relevant for some platforms,
  perhaps ARM7 (32-bit)?
  (This combo isn't CI tested.)

Closes #xxxx

---

These ones remain in brotli code (in CI it can be made a non-error via -Wno-error=vla):

In file included from ./curl/lib/content_encoding.c:36:
./brotli/x64-ucrt/usr/include/brotli/decode.h:204:34: warning: variable length array used [-Wvla]
    const uint8_t encoded_buffer[BROTLI_ARRAY_PARAM(encoded_size)],
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./brotli/x64-ucrt/usr/include/brotli/port.h:253:34: note: expanded from macro 'BROTLI_ARRAY_PARAM'
#define BROTLI_ARRAY_PARAM(name) (name)
                                 ^~~~~~
In file included from ./curl/lib/content_encoding.c:36:
./brotli/x64-ucrt/usr/include/brotli/decode.h:206:48: warning: variable length array used [-Wvla]
    uint8_t decoded_buffer[BROTLI_ARRAY_PARAM(*decoded_size)]);
                           ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
./brotli/x64-ucrt/usr/include/brotli/port.h:253:35: note: expanded from macro 'BROTLI_ARRAY_PARAM'
#define BROTLI_ARRAY_PARAM(name) (name)                                   
                                 ~^~~~~                                   
2 warnings generated.                                                     

[vszakats]

Any takers for these new Schannel -Wcast-align warnings?:

./curl/lib/vtls/schannel.c:1203:21: warning: cast from 'unsigned char *' to 'unsigned int *' increases required alignment from 1 to 4 [-Wcast-align]
    extension_len = (unsigned int *)(&alpn_buffer[cur]);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./curl/lib/vtls/schannel.c:1208:6: warning: cast from 'unsigned char *' to 'unsigned int *' increases required alignment from 1 to 4 [-Wcast-align]
    *(unsigned int *)&alpn_buffer[cur] =
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./curl/lib/vtls/schannel.c:1214:16: warning: cast from 'unsigned char *' to 'unsigned short *' increases required alignment from 1 to 2 [-Wcast-align]
    list_len = (unsigned short*)(&alpn_buffer[cur]);
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3 warnings generated.

UPDATE: Silenced them. There is chance these warnings are relevant for certain platforms (ARM7?) with particular alignment requirements, though.

[vszakats]

The casts seems fine, though I'd be happy if someone could take a hard look at them.

One possible issue the warnings may cause is for those who build with llvm/clang and -DCURL_WERROR=ON (our CI for example), and some so far hidden warning comes up. One such case is when building with Brotli, where the warning is in Brotli headers.

But, overall this patch only matches up CMake+clang builds with CMake+gcc and autotools ones, where warnings were enabled all along.

[vszakats]

Small update: The schannel.c warnings were already visible prior to this patch in autotools with --enable-warnings --with-schannel. (Not in CI runs though.)