Add support for a configure script to generate config.mk

* Remove old static config.mk
* Add configure script that generates a config.mk

This commit adds a very simple configure script to pimd that relies on the
output of the uname tool to generate config.mk instead.  With this configure
script it is also possible to enable other features of pimd as well:

  --enable-broken-crc    Interop with older broken Cisco firmware that did the
                         checksum in PIM Register messages slightly wrong.
                         Should not be needed anymore.
  --enable-old-kernel    Really old UNIX kernels may need this for PIM Register
                         messages.  Should not be needed anymore.
  --enable-kernel-encap  Some kernels can do the PIM Register encapsulation by
                         themselves, this speeds up processing considerably.
                         Only enable this if your kernel supports it.
  --enable-kernel-mfc    Some specialized kernels may have (*,G) MFC support.
                         Only enable this if your kernel supports it.
  --enable-memory-save   Save 4 bytes per unconfigured interface per routing
                         entry.  Must restart pimd and cause routing table to
                         be flushed when configuring new interfaces.
  --enable-scoped-acls   Scoped access control list support in pimd.conf, as
                         phyint IFNAME [scoped <MCGROUP> masklen <LEN>], e.g.
                         phyint fxp0 scoped 239.0.0.0 masklen 8
  --enable-rsrr          Routing Support for Resource Reservations, currently
                         used by RSVP.  EXPERIMENTAL
  --with-includes=PATH   Use this if the multicast header files are not in the
                         standard location on your system. E.g., /sys

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>