Recently I'm seeing cases where "gcc -E" mysteriously omits blank
lines, (even though it prints the blank lines in other very similar
cases). Rather than trying to decipher and imitate this, just get rid
of the blank lines.

This approach with sed to kill the lines before the diff is better
than "diff -B" since when there is an actual difference, the presence
of blank lines won't make the diff harder to read.