Simplify Rust conditionals

We can greatly simplify the Rust goop now since:

1. upstream now hard requires Rust, so there's no need to make a
   conditional for `--enable-rust`.
2. we're not rebasing rpm-ostree in f27, so we can simplify the `>= 28`
   conditionals
3. el7 is strictly backports only, so we can simplify the `0%{?rhel}`
   conditionals (though we still want to support CentOS 7)
4. the new recommended way of using Rust in el8 is to just
   `BuildRequires: rust-toolset`, so we can drop all the SCL crud.

rpm-ostree.spec

@@ -1,16 +1,3 @@
-# Upstream has --enable-rust, but let's use it by default in Fedora
-# Note the Rust sources are in the tarball using cargo-vendor.
-# For RHEL > 7 we need the toolset.
-%if 0%{?fedora} >= 28 || 0%{?rhel} > 7
-    %bcond_without rust
-    %if 0%{?rhel} > 7
-        %define rusttoolset_version rust-toolset-1.26
-        %define rusttoolset scl enable %{rusttoolset_version} --
-    %endif
-%else
-    %bcond_with rust
-%endif
-
 Summary: Hybrid image/package system
 Name: rpm-ostree
 Version: 2018.9
@@ -25,25 +12,23 @@ URL: https://github.com/projectatomic/rpm-ostree
 Patch0: 0001-rust-Drop-crates-io-patch-and-use-0.4.0.patch
 Patch1: 0001-compose-Don-t-require-SELinux-policy-in-legacy-path.patch
 
-%if %{with rust}
 %if !%{defined rust_arches}
 # It's not defined yet in the base CentOS7 root
 %define rust_arches x86_64 i686 armv7hl aarch64 ppc64 ppc64le s390x
 %endif # defined rust_arches
+
 ExclusiveArch: %{rust_arches}
-    %if %{defined rusttoolset_version}
+
-BuildRequires: %{rusttoolset_version}-cargo
+%if 0%{?fedora}
-BuildRequires: %{rusttoolset_version}-rust
-BuildRequires: %{rusttoolset_version}-runtime
-    %else
-        # This one is only in Fedora, we're not actually using it right now
-        # but we may in the future.
-        %if 0%{?fedora} >= 28
 BuildRequires: rust-packaging
-        %endif
+%else
+%if 0%{?rhel} < 8
+# really, this is for CentOS 7 (CAHC)
 BuildRequires: cargo
-    %endif # defined rusttoolset_version
+%else
-%endif # with_rust
+BuildRequires: rust-toolset
+%endif
+%endif
 
 # For the autofiles bits below
 BuildRequires: /usr/bin/python3
@@ -133,16 +118,12 @@ The %{name}-devel package includes the header files for %{name}-libs.
 %autosetup -Sgit -n %{name}-%{version}
 
 %build
-%{?rusttoolset} env NOCONFIGURE=1 ./autogen.sh
+env NOCONFIGURE=1 ./autogen.sh
-# Override the invocation of ./configure, since %%configure is multi-line, we
+%configure --disable-silent-rules --enable-gtk-doc
-# can't just prefix it with scl enable.
+make %{?_smp_mflags}
-%define _configure %{?rusttoolset} ./configure
-%configure --disable-silent-rules --enable-gtk-doc \
-           %{?with_rust:--enable-rust}
-%{?rusttoolset} make %{?_smp_mflags}
 
 %install
-%{?rusttoolset} make install DESTDIR=$RPM_BUILD_ROOT INSTALL="install -p -c"
+make install DESTDIR=$RPM_BUILD_ROOT INSTALL="install -p -c"
 find $RPM_BUILD_ROOT -name '*.la' -delete
 
 # I try to do continuous delivery via rpmdistro-gitoverlay while
@@ -202,6 +183,9 @@ $PYTHON autofiles.py > files.devel \
 %files devel -f files.devel
 
 %changelog
+* Tue Dec 04 2018 Jonathan Lebon <jonathan@jlebon.com>
+- Simplify Rust conditionals
+
 * Fri Nov 02 2018 Jonathan Lebon <jonathan@jlebon.com> - 2018.9-3
 - Backport patch for https://pagure.io/dusty/failed-composes/issue/956