Revert "Don't use AC_CANONICAL_TARGET"

This reverts commit cd98eb15cb.

It turns out that we just started using AX_PROG_CC_FOR_BUILD, which
for some reason requires AC_CANONICAL_TARGET. That seems wrong to
me, but for now, lets just keep using it.

configure.ac

@@ -30,6 +30,7 @@ AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_AUX_DIR([build-aux])
 
 AC_CANONICAL_HOST
+AC_CANONICAL_TARGET
 
 # Define a string for the earliest version that this release has
 # backwards binary compatibility with for all interfaces a module