Look for correct "ar" to cross-compile

For some reason, autoconf doesn't by default have an "AC_PROG_AR" for
this -- possibly it's assumed that any "ar" you have will work
everyplace.

Fixes bug 3909; found by sid77.

This fixes a build issue first present in fdbdb4dc, but the bug
(of not using a correct ar) has been in every Tor version ever: it
just didn't matter until then.

changes/bug3909

+  o Build fixes:
+    - Search for a platform-specific version of "ar" when cross-compiling.
+      Should fix builds on iOS. Found by Marco Bonetti.

configure.in

@@ -118,6 +118,12 @@ if test "$enable_local_appdata" = "yes"; then
             [Defined if we default to host local appdata paths on Windows])
 fi
 
+dnl check for the correct "ar" when cross-compiling
+AN_MAKEVAR([AR], [AC_PROG_AR])
+AN_PROGRAM([ar], [AC_PROG_AR])
+AC_DEFUN([AC_PROG_AR], [AC_CHECK_TOOL([AR], [ar], [ar])])
+AC_PROG_AR
+
 AC_PROG_CC
 AC_PROG_CPP
 AC_PROG_MAKE_SET