Allow Zs characters to be composed

* src/composite.c (char_composable_p): Allow SPC and other Zs
characters to be composed.

* lisp/composite.el (compose-gstring-for-graphic): Don't
reject characters whose general category is Zs.  (Bug#14461)
1 job for master in 90 minutes (queued for 1 second)
Status Job ID Name Coverage
failed #3216


Name Stage Failure
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds