Use pkg-config if we can. Clean up some CMake stuff.

Use pkg-config if we can.  Clean up some CMake stuff.

If we have pkg-config, *and* it has .pc files for libpcap, use it to get
the C compiler flags and linker flags for libpcap.

find_library() sets a cache variable; when we're looping over libraries,
trying to find their full paths, we really want the variable to act as a
local variable, as we're looking up different libraries, so unset it
after we're finished processing a particular library.

When we're searching for static libraries, save the current value of
CMAKE_FIND_LIBRARY_SUFFIXES, set it to ".a", and then restore it when
we're done.  Don't use cmake_push_check_state() for that, as
CMAKE_FIND_LIBRARY_SUFFIXES is *not* one of the variables that it's
guaranteed to save and restore.