make provision will fail if "sed" was installed with Homebrew

[tolleiv]

Hi,

the check "if [[ "$(uname -s)" == "Darwin" ]]; then" within the ./provision/prepare-provision will decide wrong if "sed" was installed with Homebrew.

Eventually checking the version would be an option.
Here's what I get with the gnu-sed from Homebrew:

$ sed --version
sed (GNU sed) 4.2.2
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Jay Fenlason, Tom Lord, Ken Pizzini,
and Paolo Bonzini.
GNU sed home page: <http://www.gnu.org/software/sed/>.
General help using GNU software: <http://www.gnu.org/gethelp/>.
E-mail bug reports to: <bug-sed@gnu.org>.
Be sure to include the word ``sed'' somewhere in the ``Subject:'' field.

Cheers