boringssl.sh: fix deprecation warning inside clang

```
/usr/lib/llvm-19/lib/clang/19/include/mm3dnow.h:16:2: error: "The <mm3dnow.h> header is deprecated, and 3dNow! intrinsics are unsupported. For other intrinsics, include <x86intrin.h>, instead." [-Werror,-W#warnings]
   16 | #warning "The <mm3dnow.h> header is deprecated, and 3dNow! intrinsics are unsupported. For other intrinsics, include <x86intrin.h>, instead."
      |  ^
1 error generated.
```
https://github.com/curl/curl-for-win/actions/runs/11714432529/job/32629108727#step:3:4286

Author: vszakats

boringssl.sh

@@ -66,6 +66,7 @@ _VER="$1"
   [ "${CW_DEV_INCREMENTAL:-}" != '1' ] && rm -r -f "${_PKGDIR:?}" "${_BLDDIR:?}"
 
   CFLAGS="-ffile-prefix-map=$(pwd)="
+  CPPFLAGS=''
   LIBS='-lpthread'  # for tests
   options=''
 
@@ -107,6 +108,8 @@ _VER="$1"
       options+=' -DDISABLE_GO=ON'
       options+=' -DDISABLE_PERL=ON'
 
+      CPPFLAGS+=' -D_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS'
+
       # Patch out to avoid redefinition errors
       sed -i.bak 's/-D_WIN32_WINNT=_WIN32_WINNT_WIN7//g' ./CMakeLists.txt
     else
@@ -127,8 +130,8 @@ _VER="$1"
     # shellcheck disable=SC2086
     cmake -B "${_BLDDIR}" ${_CMAKE_GLOBAL} ${_CMAKE_CXX_GLOBAL} ${options} \
       '-DBUILD_SHARED_LIBS=OFF' \
-      "-DCMAKE_C_FLAGS=${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${_LDFLAGS_GLOBAL} ${LIBS}" \
-      "-DCMAKE_CXX_FLAGS=${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${_LDFLAGS_GLOBAL} ${LIBS} ${_CXXFLAGS_GLOBAL} ${_LDFLAGS_CXX_GLOBAL}"
+      "-DCMAKE_C_FLAGS=${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${CPPFLAGS} ${_LDFLAGS_GLOBAL} ${LIBS}" \
+      "-DCMAKE_CXX_FLAGS=${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${CPPFLAGS} ${_LDFLAGS_GLOBAL} ${LIBS} ${_CXXFLAGS_GLOBAL} ${_LDFLAGS_CXX_GLOBAL}"
   fi
 
   cmake --build "${_BLDDIR}"  # --verbose