audio/libsidplay: unbreak build with Clang 6 (C++14 by default)

configure:4036: c++ -o conftest -O2 -pipe -O3 -fno-strict-aliasing -fno-omit-frame-pointer -march=native -fstack-protector -fstack-protector conftest.cc >&5
In file included from conftest.cc:32:
In file included from /usr/include/c++/v1/stdlib.h:94:
/usr/include/stdlib.h:97:1: error: function declared 'noreturn' after its first declaration
_Noreturn void exit(int);
^
/usr/include/sys/cdefs.h:280:22: note: expanded from macro '_Noreturn'
#define _Noreturn noreturn

^

conftest.cc:11:6: note: declaration missing 'noreturn' attribute is here
void exit (int);

^

[...]
configure:4053: error: cannot compute sizeof (char), 77

samples.cpp:80:2: error: constant expression evaluates to 128 which cannot be narrowed to type 'sbyte' (aka 'signed char') [-Wc++11-narrowing]

0x80,0x91,0xa2,0xb3,0xc4,0xd5,0xe6,0xf7,
^~~~

samples.cpp:80:2: note: insert an explicit cast to silence this issue

0x80,0x91,0xa2,0xb3,0xc4,0xd5,0xe6,0xf7,
^~~~
static_cast<sbyte>( )

PR: 224947
Reported by: pkg-fallout (blocks 8 ports)
Submitted by: tijl (autoreconf)
Approved by: portmgr blanket