build: Fix DPKG_BUILD_RELEASE_DATE to require DPKG_PROG_PERL

It uses perl.

debian/changelog

@@ -7,6 +7,7 @@ dpkg (1.18.14) UNRELEASED; urgency=medium
     - Fix inversion logic in libmd configure check when requested explicitly.
     - Do not consider the compressor libraries available in configure if the
       headers are not usable.
+    - Fix DPKG_BUILD_RELEASE_DATE to require DPKG_PROG_PERL, as it uses perl.
 
   [ Updated man pages translations ]
   * German (Helge Kreutzmann).

m4/dpkg-build.m4

@@ -13,6 +13,7 @@ AC_DEFUN([DPKG_BUILD_SHARED_LIBS], [
 # DPKG_BUILD_RELEASE_DATE()
 # -----------------------
 AC_DEFUN([DPKG_BUILD_RELEASE_DATE], [
+  AC_REQUIRE([DPKG_PROG_PERL])
   TIMESTAMP=$(PERL=$PERL $srcdir/run-script scripts/dpkg-parsechangelog.pl -l$srcdir/debian/changelog -STimestamp)
   PACKAGE_RELEASE_DATE=$(date --utc --date="@$TIMESTAMP" +%Y-%m-%d)
   AC_SUBST([PACKAGE_RELEASE_DATE])