The newest versions of CMake require outputs of custom commands be in
DEPENDS rather than SOURCES. The SOURCES just end up looking on the
filesystem and not finding anything. Makes for very hard to debug build
errors. =/

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@207722 91177308-0d34-0410-b5e6-96231b3b80d8