...except OSX where the compile option is not available

OS X versioning of clang is completely different. Let's make the
user decide.

- The bug https://llvm.org/bugs/show_bug.cgi?id=23490 should be fixed then.
- Added --enable-fastparser=force option to override the detection.

Copy PKG_CHECK_VARS from pkg.m4 from pkg-config 0.28 to m4/ax_pkg_check_vars.m4 so we have PKG_CHECK_VARS available

Remove version-script support to remove additional complexity it introduces (it's not really needed when we hide internal symbols)

Merge libknot-int and libknot-yparser into libknot with versioned symbols to separate main KNOT library and INTERNAL+YPARSER

1. libknot - interfaces has been added (0:0:0 -> 1:0:1)
2. libzscanner - source change, but interface has not (0:1:0)
3. libdnssec - new library (0:0:0)
4. libknot-int - new library (0:0:0)
5. libknot-yparser - new library (0:0:0)

As of now, the version of the library is consistent with the release.

This is a problem with packaging, when the binary is compiled on
a machine supporting newer SSE extensions than the box where the
package is installed. Moreover, CFLAGS decisions should be up to
maintainer or the one building the software.

Reverted to usual recvmmsg/sendmmsg availability check.

(This implementation has EXTRA-FLAGS option.)

norefs

Change-Id: I456b89ce8a7b2624b0196013527682fce13078e6

Change-Id: I429b3833c5db9e78220beb1ea72980b557ba06c4

Change-Id: I77615a328dce98e39964c567b55cf7c4dca331e2

http://www.freebsd.org/cgi/query-pr.cgi?pr=165798

This reverts commit 68960102.

refs #631