maketgz: fix sed issues on OSX

maketgz creates release tarballs and removes the -DEV string in curl
version (e.g. 7.58.0-DEV), else -DEV shows up on command line when curl
is run. maketgz works fine on linux but fails on OSX. Problem is with
the sed commands that use option -i without an extension. Maketgz
expects GNU sed instead of BSD and this simply won't work on OSX. Adding
a backup extension .bak after -i fixes this issue

Running the script as if on OSX gives this error:

sed: -e: No such file or directory

Adding a .bak extension resolves it

Closes #2660

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