curl: silence false positives with linux cross-build from mac [ci skip]

This has been discussed earlier with the curl team and concluded
these are false positives. Silencing them in code is not practical,
and it only affects a narrow build case.

Therefore this patch silences it within the narrow build case by
making this warning a non-error.

This makes it possible to enable `werror` by default.

```
curl/lib/vquic/curl_ngtcp2.c: In function 'cf_ngtcp2_send':
curl/lib/vquic/curl_ngtcp2.c:1511:38: error: potential null pointer dereference [-Werror=null-dereference]
 1511 |     nva[i].value = (unsigned char *)e->value;
      |                                     ~^~~~~~~
curl/lib/vquic/curl_ngtcp2.c:1510:23: error: potential null pointer dereference [-Werror=null-dereference]
 1510 |     nva[i].namelen = e->namelen;
      |                      ~^~~~~~~~~
curl/lib/vquic/curl_ngtcp2.c:1509:37: error: potential null pointer dereference [-Werror=null-dereference]
 1509 |     nva[i].name = (unsigned char *)e->name;
      |                                    ~^~~~~~
curl/lib/vquic/curl_ngtcp2.c:1512:24: error: potential null pointer dereference [-Werror=null-dereference]
 1512 |     nva[i].valuelen = e->valuelen;
      |                       ~^~~~~~~~~~
```
Ref: https://github.com/curl/curl-for-win/actions/runs/16525969694/job/46739239259#step:3:4981

(same for curl_quiche.c, curl_osslq.c, curl_msh3.c)

Ref: curl/curl#18025 (not merged)
Ref: curl/curl#12079 (not merged)
Ref: curl/curl#12076

Author: vszakats

curl.sh

@@ -31,6 +31,7 @@ _VER="$1"
   # Build
 
   options=''
+  CFLAGS=''
   CPPFLAGS=''
 
   LIBS=''
@@ -321,6 +322,16 @@ _VER="$1"
     options+=' -DCMAKE_UNITY_BUILD=ON'
     if [[ "${_CONFIG}" =~ (dev|unitybatch) ]]; then
       options+=' -DCMAKE_UNITY_BUILD_BATCH_SIZE=30'
+    else
+      # Prevent false positive compiler warnings breaking the build when
+      # building for linux on mac in single-batch unity mode.
+      # Seen with gcc 9.2.0 via filosottile/musl-cross/musl-cross
+      if [ "${_HOST}" = 'mac' ] && \
+         [ "${_OS}" = 'linux' ] && \
+         [ "${_CC}" = 'gcc' ] && \
+         [ "${_CRT}" = 'musl' ]; then
+        CFLAGS+=' -Wno-error=null-dereference'
+      fi
     fi
   fi
 
@@ -375,7 +386,7 @@ _VER="$1"
       -DBUILD_STATIC_LIBS=ON \
       -DCURL_HIDDEN_SYMBOLS=ON \
       -DCMAKE_RC_FLAGS="${_RCFLAGS_GLOBAL}" \
-      -DCMAKE_C_FLAGS="${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CPPFLAGS} ${_LDFLAGS_GLOBAL}" \
+      -DCMAKE_C_FLAGS="${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${CPPFLAGS} ${_LDFLAGS_GLOBAL}" \
       -DCMAKE_EXE_LINKER_FLAGS="${LDFLAGS} ${LDFLAGS_BIN} ${LIBS}" \
       -DCMAKE_SHARED_LINKER_FLAGS="${LDFLAGS} ${LDFLAGS_LIB} ${LIBS}" \
       || { cat "${_BLDDIR}"/CMakeFiles/CMake*.yaml; false; }