CryptoPkg: Fix build problem with XCODE
    
    REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1089
    
    When building CryptoPkg with XCODE, blow error may result
    
    * usr/lib/clang/9.0.0/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: Jian Wang <jian.j.wang@intel.com>
    Cc: Ting Ye <ting.ye@intel.com>
    Signed-off-by: Xiaoyu Lu <xiaoyux.lu@intel.com>
    Reviewed-by: Jian J Wang <jian.j.wang@intel.com>