contrib: integrate RPM spec

Remove OpenSUSE bug report link, set version to @VERSION@, make it build with
CentOS 8 etc.

Related: OS#4550
Change-Id: I5376fde96c7442eb70f49a2ce38f57a817a94f81
diff --git a/configure.ac b/configure.ac
index f7580e6..6962261 100644
--- a/configure.ac
+++ b/configure.ac
@@ -53,4 +53,5 @@
 AC_SUBST([regular_CPPFLAGS])
 AC_SUBST([regular_CFLAGS])
 AC_CONFIG_FILES([Makefile src/Makefile include/Makefile include/libgtpnl/Makefile include/linux/Makefile tools/Makefile libgtpnl.pc])
-AC_OUTPUT
+AC_OUTPUT(
+	contrib/libgtpnl.spec)