pkg-autotools: fix patching libtool for version 2.4

If the libtool used by the package is 2.4 (i.e. with no patchlevel), we
end up with error messages like this one, seen when building ntp-4.2.8p3:

    /bin/sh: line 0: test: 2.4: integer expression expected

That's because the current sed expression "[0-9].[0-9]." only deletes content
like "2.4.", so it leaves "2.4" (no patch level *and* no second period) intact,
resulting in an ltmain_patchlevel of "2.4", not empty, and not "0".  Then the
shell line goes bad, because the built in shell test -gt operates on integers
only, and fails on floating point numbers (like "2.4").

Additionally, the existing sed lines are problematic in other ways, because the
current expression "[0-9].[0-9]" will match "234" as well as "2.4", as an unescaped
period matches any character, not just a period.  Also, the lack of an asterisk
after the first character may be a problem in the future, if a two digit initial
number is used.

So, this patch changes the sed regex as follows:

* Match a string of digits when searching for a number.  I.e. "[0-9]*"
* Match specifically a period in between two numbers.  I.e. "\."
* When searching for a patch level, make the second period optional.  I.e. "\.*"

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>