Replace %{rhel} tests with more general %{have_ublk} macro

Also force ./configure --enable-ublk or --disable-ublk option.

libnbd.spec

@@ -3,6 +3,11 @@
 %global have_ocaml 1
 %endif
 
+# No ublk in RHEL 9.
+%if !0%{?rhel}
+%global have_ublk 1
+%endif
+
 # If we should verify tarball signature with GPGv2.
 %global verify_tarball_signature 1
 
@@ -51,7 +56,7 @@ BuildRequires:  libxml2-devel
 # For nbdfuse.
 BuildRequires:  fuse3, fuse3-devel
 
-%if !0%{?rhel}
+%if 0%{?have_ublk}
 # For nbdublk
 BuildRequires:  liburing-devel >= 2.2
 BuildRequires:  ubdsrv-devel >= 1.0-3.rc6
@@ -182,7 +187,7 @@ Recommends:     fuse3
 This package contains FUSE support for %{name}.
 
 
-%if !0%{?rhel}
+%if 0%{?have_ublk}
 %package -n nbdublk
 Summary:        Userspace NBD block device
 Requires:       %{name}%{?_isa} = %{version}-%{release}
@@ -233,7 +238,13 @@ autoreconf -i
 %endif
     --enable-fuse \
     --disable-golang \
-    --disable-rust
+    --disable-rust \
+%if 0%{?have_ublk}
+    --enable-ublk \
+%else
+    --disable-ublk \
+%endif
+    %{nil}
 
 make %{?_smp_mflags}
 
@@ -252,7 +263,9 @@ rm $RPM_BUILD_ROOT%{_mandir}/man3/libnbd-golang.3*
 rm $RPM_BUILD_ROOT%{_mandir}/man3/libnbd-ocaml.3*
 %endif
 
-%if 0%{?rhel}
+# Remove this section in libnbd >= 1.20, since upstream was fixed to
+# not install nbdublk completion if nbdublk was not built.
+%if !0%{?have_ublk}
 # Delete nbdublk on RHEL.
 rm $RPM_BUILD_ROOT%{_datadir}/bash-completion/completions/nbdublk
 %endif
@@ -357,7 +370,7 @@ make %{?_smp_mflags} check || {
 %{_mandir}/man1/nbdfuse.1*
 
 
-%if !0%{?rhel}
+%if 0%{?have_ublk}
 %files -n nbdublk
 %{_bindir}/nbdublk
 %{_mandir}/man1/nbdublk.1*
@@ -371,7 +384,7 @@ make %{?_smp_mflags} check || {
 %{_datadir}/bash-completion/completions/nbdfuse
 %{_datadir}/bash-completion/completions/nbdinfo
 %{_datadir}/bash-completion/completions/nbdsh
-%if !0%{?rhel}
+%if 0%{?have_ublk}
 %{_datadir}/bash-completion/completions/nbdublk
 %endif