Log Message:
pkgtools/pkgin: Fix header file checks in configure.

https://github.com/NetBSDfr/pkgin/issues/133

   configure.ac wrongly checks for header files in build environment

   When configuring with --with-libarchive=/usr/pkg, pkgin's configure
   script tests whether /usr/pkg and /usr/pkg/include/archive.h exist.

   This is wrong -- it doesn't matter whether
   /usr/pkg/include/archive.h exists; what matters is whether the
   compiler can resolve #include <archive.h>, which it might be doing
   relative to a sysroot like
   /home/riastradh/netbsd/current/obj.evbarm64/destdir.evbarm.

   configure.ac should use AC_CHECK_HEADER instead to check whether
   #include <archive.h> works, and AC_CHECK_LIB to check whether
   linking with -larchive works; similarly for libfetch, openssl, and
   sqlite3. It should also provide a way to specify other arguments
   than just CPPFLAGS=-I/prefix/include LDFLAGS=-L/prefix/lib
   LIBS=-lwhatever, e.g. in principle some environments may need
   LDFLAGS=-L/sdk/prefix/lib\ -Wl,-R/opt/prefix/lib or LIBS=-lfetch
   -lssl -lcrypto.

   The result of the status quo is that pkgin can't currently be
   reasonably cross-compiled.