Fix build on clang 5 (dotnet/corefx#27178)

Fix build on clang 5 (dotnet/corefx#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.

Commit migrated from https://github.com/dotnet/corefx/commit/a9f4a46f48baeaadd515d85ac9de1249b8023a5a