ghostscript: Pin to C17 std

ghostscript: Pin to C17 std

The code defines a custom 'bool' type (as an 'int'), which is incompatible
with C23 in which bool is a keyword, and trying to use <stdbool.h> fails
because 'int' and 'bool' are used interchangeably in the code.

Add the flag to CC variable, since CFLAGS is used by both c and c++ compilers
and clang++ is less forgiving when C compiler only option is used on its
cmdline so it complains about -std=gnu17 and bails out.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>