register keyword removed from C++17

[tonycoz]

(Same as pmqs/Compress-Raw-Bzip2#11 )

As you know we build blead perl using C++, but some modern C++ compilers that default to C++17 report an error for the register keyword:

 c++ -DNO_MATHOMS -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -Werror=pointer-arith -Werror=vla -Wextra -Wno-long-long -Wno-declaration-after-statement -Wc++-compat -Wwrite-strings -O2   -DVERSION=\"2.204\" -DXS_VERSION=\"2.204\" -fPIC "-I../.."  -DBZ_NO_STDIO  compress.c
compress.c:190:13: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
            register UChar  rtmp;
            ^~~~~~~~~

This can be fixed by simply removing "register" and if you like I can supply a PR doing that, but that will also increase the distance between your zlib source and the upstream zlib source, you might prefer to add:

#define register

at the top of deflate.c and trees.c instead.

[pmqs]

fix applied, thanks!