build: regenerate spec file after configure

Configure options may change pathnames that affect compilation, but
the files that are affected aren't always regenerated.  This is
especially relevant to files processed by var_subst.inc, such as the
rpm spec file.  To prevent issues including build failures, go ahead
and `make clean` in autogen.sh after running `configure`.

Change-Id: I26ecbe06e29e9c1821cb0d623ef0783709eefcbc
Signed-off-by: Greg Padgett <gpadgett@redhat.com>