configure.ac: Check for existence of freetype2 library.

  In preparation for using freetype2 to load .ttf files.

  <rant>
  There are two existing ttf loading libraries for Allegro 4,
    AllegroTTF and AllegroFont.  The former did not have a
    straightforward license (it claims to be "swap-ware"), so
    I rejected it outright, fearing GPLv3+ incompatibility.
  Unfortunately, the only other library, AllegroFont, turned
    out to be unmaintained, and the latest version I found
    triggered several disturbing warnings in clang's scan-build,
    the most important being that "masked_bitmap" was being
    used, but not initialized.  Source diving revealed that
    AllegroFont's code seemed to be copy-pasted repeatedly,
    meaning that extensive changes would be needed to clean up
    the masked_bitmap bug, and I would probably miss some edge
    cases during testing of my modifications.
  I am now thus resolved to create my own .ttf handling library
    for Allegro 4, bummer.
  </rant>