CryptoPkg: Fix possible build problem with Clang
    
    REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1089
    
    When building CryptoPkg with Clang, blow error may result
    
    * /usr/lib/llvm-3.8/lib/clang/3.8.0/include/stdatomic.h:105:17: error:
        unknown type name 'wchar_t'
    
    Since the C native atomics are C11 feature we can explicitly
    use C99 to work around it.
    add -std=c99 to avoid it
    
    * openssl/crypto/conf/conf_sap.c:71:12: error: variable 'ret' is
      uninitialized
      when used here [-Werror,-Wuninitialized]
    
    Suppress warnings in OpenSSL so we don't break the build with -Werror.
    add -Wno-error=uninitialized to disalbe this warning
    
    Cc: Ting Ye <ting.ye@intel.com>
    Cc: Jian J Wang <jian.j.wang@intel.com>
    Signed-off-by: Xiaoyu Lu <xiaoyux.lu@intel.com>
    Reviewed-by: Jian J Wang <jian.j.wang@intel.com>