Speed up search by 85+%

Large searches spend most of their time in converting the current line
for output. For most lines which do not match we do not need to
convert, so this patch skips that.

This patch does the following optimizations:

- Only convert when there is an actual match, or the line
contains backspaces (for backspace conversion)
- Don't do the full conversion for case independent searches, but
ask the low level search functions to be case independent instead
- Use the more optimized strstr/strcasestr functions in glibc for the
actual searching in the no regular expression case

Also regenerate the configure file to check for strcasestr. This is unfortunately most of the patch, as I didn't have the same autoconf version.