[OCaml] If compiled without --enable-shared, hide packages from toplevel.

Pretend they do not exist using exists_if. This is better than
the current situation, which is the error:

    Error: The external function `llvm_global_succ' is not available

but still somewhat confusing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220845 91177308-0d34-0410-b5e6-96231b3b80d8

bindings/ocaml/backends/META.llvm_backend.in

@@ -4,4 +4,5 @@ description = "@TARGET@ Backend for LLVM"
 requires = "llvm"
 archive(byte) = "llvm_@TARGET@.cma"
 archive(native) = "llvm_@TARGET@.cmxa"
+exists_if(toplevel) = "dllllvm.so"
 directory = "."

bindings/ocaml/llvm/META.llvm.in

@@ -3,6 +3,7 @@ version = "@PACKAGE_VERSION@"
 description = "LLVM OCaml bindings"
 archive(byte) = "llvm.cma"
 archive(native) = "llvm.cmxa"
+exists_if(toplevel) = "dllllvm.so"
 directory = "."
 
 package "analysis" (