rename ENABLE_THREADS to LLVM_ENABLE_THREADS

Now that it needs to be exported in a public header (Valgrind.h)
it should be prefixed to avoid collision with other projects.
Add it to llvm-config.h as well.

This'll require regenerating the configure script after this
commit, but I don't have the required autoconf version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145214 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/RWMutex.cpp b/lib/Support/RWMutex.cpp
index d0b1e10..d14b976 100644
--- a/lib/Support/RWMutex.cpp
+++ b/lib/Support/RWMutex.cpp
@@ -20,7 +20,7 @@
 //===          independent code.
 //===----------------------------------------------------------------------===//
 
-#if !defined(ENABLE_THREADS) || ENABLE_THREADS == 0
+#if !defined(LLVM_ENABLE_THREADS) || LLVM_ENABLE_THREADS == 0
 // Define all methods as no-ops if threading is explicitly disabled
 namespace llvm {
 using namespace sys;