cmake: really enable warnings with clang

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

1 files changed, 3 insertions, 3 deletions

diff --git a/lib/vtls/schannel.c b/lib/vtls/schannel.c
index 454eb7967..fcfb9c6df 100644
--- a/lib/vtls/schannel.c
+++ b/lib/vtls/schannel.c
@@ -1200,18 +1200,18 @@ schannel_connect_step1(struct Curl_easy *data, struct connectdata *conn,
 
     /* The first four bytes will be an unsigned int indicating number
        of bytes of data in the rest of the buffer. */
-    extension_len = (unsigned int *)(&alpn_buffer[cur]);
+    extension_len = (unsigned int *)(void *)(&alpn_buffer[cur]);
     cur += sizeof(unsigned int);
 
     /* The next four bytes are an indicator that this buffer will contain
        ALPN data, as opposed to NPN, for example. */
-    *(unsigned int *)&alpn_buffer[cur] =
+    *(unsigned int *)(void *)&alpn_buffer[cur] =
       SecApplicationProtocolNegotiationExt_ALPN;
     cur += sizeof(unsigned int);
 
     /* The next two bytes will be an unsigned short indicating the number
        of bytes used to list the preferred protocols. */
-    list_len = (unsigned short*)(&alpn_buffer[cur]);
+    list_len = (unsigned short*)(void *)(&alpn_buffer[cur]);
     cur += sizeof(unsigned short);
 
     list_start_index = cur;