Fix build on clang 5 (#27178)

This contains two fixes, both related to new warnings introduced with
clang 5:
http://releases.llvm.org/5.0.0/tools/clang/docs/ReleaseNotes.html

1. clang 5 introduces a -Wzero-as-null-pointer-constant warning, which
becomes an error with Werror. This warning is not known by older
versions of clang and affects a lot of C/C++ code, such as:

    static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

It also affects code we pick up from our dependency libraries. Lets
check if the compiler knows about this warning and disable it if so.

2. clang 5 introduces checks for casts. The library expects a `char *`,
so lets cast our pointer to the expected type so it will continue
working.

src/Native/Unix/CMakeLists.txt

@@ -259,6 +259,9 @@ add_subdirectory(System.IO.Compression.Native)
 
 add_compile_options(-Weverything)
 add_compile_options(-Wno-c++98-compat-pedantic)
+if (HAVE_ZERO_AS_NULL_POINTER_CONSTANT_FLAG)
+    add_compile_options(-Wno-zero-as-null-pointer-constant)
+endif()
 
 add_subdirectory(System.Native)
 add_subdirectory(System.Net.Http.Native)

src/Native/Unix/System.Security.Cryptography.Native/pal_ssl.cpp

@@ -609,6 +609,6 @@ extern "C" void CryptoNative_SslGet0AlpnSelected(SSL* ssl, const uint8_t** proto
 
 extern "C" int32_t CryptoNative_SslSetTlsExtHostName(SSL* ssl, const uint8_t* name)
 {
-    return static_cast<int32_t>(SSL_set_tlsext_host_name(ssl, name));
+    return static_cast<int32_t>(SSL_set_tlsext_host_name(ssl, const_cast<unsigned char*>(name)));
 }
 

src/Native/Unix/configure.cmake

@@ -1,3 +1,4 @@
+include(CheckCXXCompilerFlag)
 include(CheckCXXSourceCompiles)
 include(CheckCXXSourceRuns)
 include(CheckCSourceCompiles)
@@ -29,6 +30,13 @@ endif ()
 # We compile with -Werror, so we need to make sure these code fragments compile without warnings.
 set(CMAKE_REQUIRED_FLAGS -Werror)
 
+# This compiler warning will fail code as innocuous as:
+# static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
+# Check if the compiler knows about this warning so we can disable it
+check_cxx_compiler_flag(
+    -Wzero-as-null-pointer-constant
+    HAVE_ZERO_AS_NULL_POINTER_CONSTANT_FLAG)
+
 # in_pktinfo: Find whether this struct exists
 check_include_files(
     "sys/socket.h;linux/in.h"