Fixing the Red Hat packaging

When adding ZMQ_VERSION macros, I incorrectly removed
the PACKAGE_VERSION macro. Adding it back.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

configure.in

@@ -13,7 +13,9 @@ AC_CONFIG_MACRO_DIR(config)
 AM_CONFIG_HEADER(src/platform.hpp)
 AM_INIT_AUTOMAKE(tar-ustar dist-zip foreign)
 
-#
+# This lets us use PACKAGE_VERSION in Makefiles
+AC_SUBST(PACKAGE_VERSION)
+
 # Libtool -version-info (ABI version)
 #
 # Currently 0.0.0 ("unstable"). Don't change this unless you