build failure on older perls

[tonycoz]

https://matrix.perl-magpie.org/results/72f69666-92a0-11f1-9c9a-bd1d6e8775ea

cc -c   -fwrapv -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -O2   -DVERSION=\"1.034\" -DXS_VERSION=\"1.034\" -fPIC "-I/opt/perl-5.18.4/lib/5.18.4/x86_64-linux/CORE"  -DOS_linux   Imager.c
Imager.xs: In function 'XS_Imager__IO_raw_read':
Imager.xs:1668:21: error: 'SSize_t_MAX' undeclared (first use in this function); did you mean 'SSIZE_MAX'?
 1668 |         if (size >= SSize_t_MAX)
      |                     ^~~~~~~~~~~
      |                     SSIZE_MAX
Imager.xs:1668:21: note: each undeclared identifier is reported only once for each function it appears in
Imager.xs: In function 'XS_Imager__IO_raw_read2':
Imager.xs:1702:21: error: 'SSize_t_MAX' undeclared (first use in this function); did you mean 'SSIZE_MAX'?
 1702 |         if (size >= SSize_t_MAX)
      |                     ^~~~~~~~~~~
      |                     SSIZE_MAX
Imager.xs: In function 'XS_Imager__IO_read':
Imager.xs:1807:21: error: 'SSize_t_MAX' undeclared (first use in this function); did you mean 'SSIZE_MAX'?
 1807 |         if (size >= SSize_t_MAX)
      |                     ^~~~~~~~~~~
      |                     SSIZE_MAX
make: *** [Makefile:564: Imager.o] Error 1