docs: change expired bzip2.org to archive.org

Fixes #2396.

configure.ac

@@ -152,9 +152,9 @@ PKG_CHECK_MODULES([OPENSSL], [libcrypto], [CXXFLAGS="$OPENSSL_CFLAGS $CXXFLAGS"]
 
 # Look for libbz2, a required dependency.
 AC_CHECK_LIB([bz2], [BZ2_bzWriteOpen], [true],
-  [AC_MSG_ERROR([Nix requires libbz2, which is part of bzip2.  See http://www.bzip.org/.])])
+  [AC_MSG_ERROR([Nix requires libbz2, which is part of bzip2.  See https://web.archive.org/web/20180624184756/http://www.bzip.org/.])])
 AC_CHECK_HEADERS([bzlib.h], [true],
-  [AC_MSG_ERROR([Nix requires libbz2, which is part of bzip2.  See http://www.bzip.org/.])])
+  [AC_MSG_ERROR([Nix requires libbz2, which is part of bzip2.  See https://web.archive.org/web/20180624184756/http://www.bzip.org/.])])
 
 
 # Look for SQLite, a required dependency.

doc/manual/installation/prerequisites-source.xml

@@ -29,7 +29,8 @@
   <literal>libbz2</literal> library.  Thus you must have bzip2
   installed, including development headers and libraries.  If your
   distribution does not provide these, you can obtain bzip2 from <link
-  xlink:href="http://www.bzip.org/"/>.</para></listitem>
+  xlink:href="https://web.archive.org/web/20180624184756/http://www.bzip.org/"
+  />.</para></listitem>
 
   <listitem><para><literal>liblzma</literal>, which is provided by
   XZ Utils. If your distribution does not provide this, you can