Sync with llvm8.0 spec file

Also replace %global compat_build 0 with %bcond_with compat_build.
This will make it easier to test building the compat version of the
package and allow us to add a CI test for it.

tests/build-compat/runtest.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+set -ex
+
+dnf download --disablerepo=* --enablerepo=test-llvm --source llvm
+
+# The src.rpm is available in the directory the test run from.
+set +e
+mock --resultdir=. --old-chroot --with compat_build --rebuild *.src.rpm
+if [ $? -ne 0 ]; then
+  cat root.log
+  cat build.log
+  exit 1
+fi
+
+exit 0