liblzma: Fix compilation with PGI compiler

- sha265.c is using some C99 specific features, in particular static
array dimensions in a function parameter array (see section 6.7.5-7
of the C99 spec).  A #ifndef check was in place to prevent compilation
under MSVC but it actually needed to check for C99 compliance instead.
Even still, the C99 code fails on a few compilers (PGI being one) so for
compatibility reasons, the C99 version of the function declaration is
removed entirely, leaving only the C89 version.

- CHECK_SYMBOL_EXISTS is used to determine the presense of bswap
functions from byteswap.h.  Most compilers re-dedefine the bswap_N
functions as a __bswap_N function implemented by the compiler.  Since
bswap_N is usually defined as a macro then it's mere presence passes
the check.  Some versions of the PGI compiler though have shipped
broken headers for byteswap.h, in particular 11.3 for x64 linux
provides byteswap.h but is missing an associated bits/byteswap.h which
causes some of the bswap_N macros to be defined but broken and unusable.
The bswap_N checks have been converted to CHECK_SOURCE_COMPILES to
ensure that the bswap_N calls are actually usable and not just merely
defined.