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.