Makefile: Use a POSIX-compatible test ('==' -> '=')
Makefile: Use a POSIX-compatible test ('==' -> '=')

With dash 0.5.7:

  # make docs
  /bin/sh: 1: test: 1.3.3: unexpected operator
  /bin/sh: 1: test: 1.3.3: unexpected operator
  /bin/sh: 1: test: 1.3.3: unexpected operator
  Makefile:47: *** cannot build output//oci-runtime-spec.pdf without either pandoc or docker.  Stop.
  # command -V test
  test is a shell builtin

POSIX defines '=' for string comparison [1]; the '==' form is a
Bashism.

SHELL was added in f3fdf03 (Makefile: prefer bash, 2016-05-25, #455)
to avoid these "unexpected operator" errors, but there's no reason to
require Bash when we can make the comparison's POSIX compliant.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

laijs/specs,opencontainers/specs,wking/opencontainer-specs,opencontainers/runtime-spec,hqhq/runtime-spec,ujuc/specs,jonboulle/specs,wking/opencontainer-runtime-spec,cloudfoundry-incubator/specs,hqhq/specs