Cross-platform sed

It's quite easy to accidentally use GNU extensions in sed expressions, which means Solaris builds start failing. See https://phabricator.haskell.org/D740 for an example.

What should we do? Here are a few possibilities:

• Force use of POSIX sed with --posix flag. I haven't tested, but maybe this won't work if Solaris sed chokes on the --posix. Alternately, use $(SED) and add --posix only if it's supported
• Detect GNU sed in autoconfigure and use $(SED) in all Makefile scripts
• Stop using sed in the scripts for something else

I don't know if this admonition applies to other random POSIX tools we use.

Trac metadata

Trac field	Value
Version	7.11
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
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CC	kgardas
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