respect standard $PKG_CONFIG env var

Rather than hardcode `pkg-config`, allow people to set PKG_CONFIG to
point to a version customized for their build.
convert to LDLIBS

The standard variable for adding -l flags is LDLIBS, not LIBS.
The default Makefile rules (which this code already relies on)
will use that when linking.

This also fixes a problem where we do not want to list -l flags
in the dependency of make targets.  When you do that, make will
do a file search in /lib and /usr/lib (hardcoded) and expand the
path to those absolute files.  This breaks when you try to cross-
compile miniupnpd for other platforms.
use CPPFLAGS for -D/-I flags

The standard variable for preprocessor flags (e.g. -I and -D) is
CPPFLAGS.  The default Makefile rules already use this variable
when compiling code, so we only need to convert the name.
Merge pull request #70 from vapier/master

Linux build clean up
minissdpd/openssdpsocket.c: Fix indentation
minissdpd/openssdpsocket.c: listen on only 1 IPv4 if only 1 interface is specified
Added PCP third party option and made it's use an option.
miniupnpd: change "allow_thirdparty" PCP option to a bit flag
miniupnpd/pcpserver.c: Some IPv6 related work on PCP.
Added missing check for int_ip.
miniupnpd/pcpserver.c: Added checks for third-party allowed for it to be used.

If allowed, checking it against source address,
with inverse logic from that of non-thirdparty case.
miniupnpd/pcpserver.c: Preliminary work for PCP fw control

Added preliminary is_fw flag, and added af to getifaddr_in6. Made
option parsing follow the RFC and also made it bit more paranoid
(there were some security problems with length checks not being done
at right place all the time; simplified flow, should be easier to
verify now that it does nothing untoward).
miniupnpd/getifaddr.c: fix when IPV6 is not enabled
add Markus Stenberg
Fix formatting in areas the next commit will touch
Add support for IGD2 AddAnyPortMapping and DeletePortMappingRange

Added python bindings for both AddAnyPortMapping and DeletePortMappingRange
Don't print garbage values on failure
Don't call deletePortMapping method for deletePortMappingRange

Length of strings needs to match before doing comparison, else we can stop early on a substring of the one were trying to match.
Return 730 error where appropriate, and output helpful debug
Merge pull request #71 from arr2036/master

Add support for IGD2 AddAnyPortMapping DeletePortMappingRange