build: Use the detected perl to invoke dpkg-architecture.pl

This will help building dpkg on systems where perl is not at
/usr/bin/perl, which we were already taking into consideration for the
resulting scripts but not during build.

debian/changelog

@@ -165,6 +165,8 @@ dpkg (1.16.1) UNRELEASED; urgency=low
   * Unify somewhat dpkg-maintscript-helper --help output with other commands.
   * Add build-indep and build-arch targets as aliases for build in
     debian/rules.
+  * Use the perl interpreter found by configure to call dpkg-architecture.pl
+    in the m4 DPKG_ARCHITECTURE macro.
   * Fix possible segfault of dpkg in findbreakcycle(). LP: #733414
 
   [ Bill Allombert]

m4/dpkg-arch.m4

@@ -6,9 +6,10 @@
 # Use dpkg-architecture from the source tree to set sh_var using DEB_VAR for
 # the target architecture, to avoid duplicating its logic.
 AC_DEFUN([_DPKG_ARCHITECTURE], [
+AC_REQUIRE([DPKG_PROG_PERL])dnl
 AC_REQUIRE([AC_CANONICAL_HOST])dnl
 $2=$(cd $srcdir/scripts; \
-    PERL5LIB=$(pwd) ./dpkg-architecture.pl -t$host -q$1 2>/dev/null)
+    PERL5LIB=$(pwd) $PERL dpkg-architecture.pl -t$host -q$1 2>/dev/null)
 ])# _DPKG_ARCHITECURE
 
 # DPKG_CPU_TYPE