test/Makefile: Support non-GNU sed
This required two changes:
1. Modifying a file inplace using -i is not portable. GNU sed does
not require an argument; the absence of it meaning it should
change the file inplace. sed found on FreeBSD and probably other
BSDs' require an argument, so inplace modification should be
specified as: sed -i ''. Obviously, both -i are incompatible...
The "solution" is to specify a backup suffix and ignore this
backup file.
2. The [1addr]i command takes its argument on the next line
according to the standard (though, it's not explicitely stated,
the syntax example suggests that). GNU sed accepts to have its
argument on the same line as the command, but not FreeBSD's sed.
Jean-Sébastien Pédron