build: fixed PREFIX usage

This change fixes the use of PREFIX by not hard wiring /usr into the path.

Besides that if no PREFIX is provided by the user, it is set to /usr/local now
which is considered to be the right location for most LSB conforming distros if
the user is compiling from sources to not interfere with the package management
for binary packages which are build using /usr as prefix in most of the cases.

current behavior without setting PREFIX will most likely interfere
with most distro package managers when building from source:
------------------------------------------------------------------------
$ sudo make uninstall
  RM    /usr/sbin/netsniff-ng
  RM    /usr/share/man/man8/netsniff-ng.8.gz
  RM    /usr/sbin/trafgen
  RM    /usr/share/man/man8/trafgen.8.gz
  RM    /usr/sbin/astraceroute
  RM    /usr/share/man/man8/astraceroute.8.gz
  RM    /usr/sbin/ifpps
  RM    /usr/share/man/man8/ifpps.8.gz
  RM    /usr/sbin/bpfc
  RM    /usr/share/man/man8/bpfc.8.gz
  RM    /usr/sbin/curvetun
  RM    /usr/share/man/man8/curvetun.8.gz
------------------------------------------------------------------------

current behavior when setting PREFIX=/usr/local looks kind of weird
because of the hard wired /usr:
------------------------------------------------------------------------
$ sudo make PREFIX=/usr/local uninstall
  RM    /usr/local/usr/sbin/netsniff-ng
  RM    /usr/local/usr/share/man/man8/netsniff-ng.8.gz
  RM    /usr/local/usr/sbin/trafgen
  RM    /usr/local/usr/share/man/man8/trafgen.8.gz
  RM    /usr/local/usr/sbin/astraceroute
  RM    /usr/local/usr/share/man/man8/astraceroute.8.gz
  RM    /usr/local/usr/sbin/ifpps
  RM    /usr/local/usr/share/man/man8/ifpps.8.gz
  RM    /usr/local/usr/sbin/bpfc
  RM    /usr/local/usr/share/man/man8/bpfc.8.gz
  RM    /usr/local/usr/sbin/curvetun
  RM    /usr/local/usr/share/man/man8/curvetun.8.gz
------------------------------------------------------------------------

using the proposed fix without setting PREFIX:
------------------------------------------------------------------------
$ sudo make uninstall
  RM    /usr/local/sbin/netsniff-ng
  RM    /usr/local/share/man/man8/netsniff-ng.8.gz
  RM    /usr/local/sbin/trafgen
  RM    /usr/local/share/man/man8/trafgen.8.gz
  RM    /usr/local/sbin/astraceroute
  RM    /usr/local/share/man/man8/astraceroute.8.gz
  RM    /usr/local/sbin/ifpps
  RM    /usr/local/share/man/man8/ifpps.8.gz
  RM    /usr/local/sbin/bpfc
  RM    /usr/local/share/man/man8/bpfc.8.gz
  RM    /usr/local/sbin/curvetun
  RM    /usr/local/share/man/man8/curvetun.8.gz
------------------------------------------------------------------------

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
built_in: changed to use RUNTIME_PAGE_SIZE instead of PAGE_SIZE

References:

https://github.com/netsniff-ng/netsniff-ng/commit/453f6eb9d79dd5aa2812ef956b22723f0a493086
https://github.com/netsniff-ng/netsniff-ng/pull/112

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
built_in: changed RUNTIME_PAGE_SIZE to use sysconf(_SC_PAGE_SIZE) instead of getpagesize()

getpagesize(2) is considered legacy and has been dropped with POSIX.1-2001.

See discussion: https://github.com/netsniff-ng/netsniff-ng/pull/112

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>