Drop dependency on srfi-13
Drop dependency on srfi-13

srfi-13 was being used in `build.scm` but not declared as a
dependency.  This change simply replaces srfi-13, which was only used
for `string-concatenate`, with `string-intersperse` from
`chicken.string`.

Fixes
http://salmonella-linux-x86.call-cc.org/master-debugbuild/gcc/linux/x86/2021/03/28/salmonella-report/install/posix-shm.html

iraikov/chicken-posix-shm