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: I453ff45ec449834dfee298ecc6015e6d54768bea

.gitignore

@@ -36,3 +36,5 @@
 /config.sub
 /m4/libtool.m4
 /m4/lt*.m4
+
+contrib/libusrp.spec

configure.ac

@@ -110,4 +110,4 @@ AC_CONFIG_FILES([
   fpga/rbf/rev2/Makefile
   fpga/rbf/rev4/Makefile
 ])
-AC_OUTPUT
+AC_OUTPUT([contrib/libusrp.spec])

contrib/libusrp.spec.in

@@ -13,14 +13,10 @@
 # license that conforms to the Open Source Definition (Version 1.9)
 # published by the Open Source Initiative.
 
-# Please submit bugfixes or comments via https://bugs.opensuse.org/
-#
-
-
 %define sover   -3_4_4+git_20190808-0
 %define libname libusrp%{sover}
 Name:           libusrp
-Version:        3.4.4+git.20190808
+Version:        @VERSION@
 Release:        0
 Summary:        Stand-alone libusrp for USRP1 from old gnuradio.git
 License:        GPL-3.0-or-later