configure.ac: don't recommend using tar/gunzip to unpack a zip.

* configure.ac:
  Add note to update build/ac-macros/sqlite.m4 if changing
  SQLITE_URL's file extension.

* build/ac-macros/sqlite.m4:
  Recommend using unzip since SQLITE_URL ends in .zip.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1469513 13f79535-47bb-0310-9956-ffa450edef68