bashism not checked for: [[

[Greg Troxel <gdt%lexort.com@localhost>]

In doing an update of proj, I came across

  if [[ stuff ]]; then

in a Makefile, that were there only one [, as POSIX requires, would have
been fine.

I know that in the case of test == one gets an error (with
PKG_DEVELOPER).

So if someone who knows the CHECK_PORTABILITY_* stuff would like to add
a test for test [[ (that only triggers on shell fragments, in scripts or
makefiles, maybe), that would be great.