build: Call AM_PROG_AR

Even though we use libtool to link static libraries, it still requires
AM_PROG_AR to be called to detect the archiver to use.

Warned-by: automake

configure.ac

@@ -22,6 +22,7 @@ DPKG_DIST_CHECK([test "$GMSGFMT" = ":" && test "$USE_NLS" = "yes"],
 # Shared libraries are disabled on purpose, currently there is no ABI stability
 # guarantee, and it will be broken at will. The infrastructure is in place just
 # to be able to test that its future activation will work.
+AM_PROG_AR
 LT_INIT([disable-shared])
 DPKG_BUILD_SHARED_LIBS
 DPKG_LINKER_VERSION_SCRIPT

debian/changelog

@@ -110,6 +110,7 @@ dpkg (1.18.0) UNRELEASED; urgency=low
       Based on a patch by Alex Potapenko <opotapenko@gmail.com>.
     - Use single suffix rules instead of non-portable %-pattern rules.
     - Pass CC to the test suite, so that we can use a non-gcc compiler.
+    - Call AM_PROG_AR to detect the correct system archiver to use.
   * Packaging:
     - Remove old trigger related Breaks and Conflicts from dpkg.
     - Only use stackprotectorstrong when building dpkg with gcc >= 4.9.