memdebug build, 'make' no longer builds the demo tools

* February 16, 2004

- Modified ares_private.h to include libcurl's memory debug header if

  CURLDEBUG is set. This makes all the ares-functions supervised properly by

  the curl test suite. This also forced me to add inclusion of the

  ares_private.h header in a few more files that are using some kind of

  memory-related resources.

- Made the makefile only build ahost and adig if 'make demos' is used.

* February 10, 2004

- Dirk Manske made ares_version.h installed with 'make install'