MFH: r490052

devel/synfig: Switch to textproc/gsed for build

The configure process for synfig uses GNU extensions (\s) in sed,
which currently get interpreted as an escape of an ordinary character.
Escapes of an ordinary character will be disallowed in future
versions of regex(3), so switch to gsed in advance to also do the
right thing.

As an aside, the \s usage may be potentially patched out, but submitter
opted to switch to gsed for now to ease maintenance burden. A later
run will be done to switch ports depending on gsed for build back
to sed when our sed becomes capable.

PR: 233438
Submitted by: kevans
Approved by: woodsb02 (maintainer)
Approved by: portmgr (miwi)