Add support for building subzero with LLVM

tools/Makefile

Index: tools/Makefile
diff --git a/tools/Makefile b/tools/Makefile
index 494ea562ed0188c87aec7f7f3eeaa981bfe03619..b9e7004380657768a3c894a055c115272d228af6 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -18,7 +18,8 @@ ifneq ($(CLANG_SRC_ROOT),)
 else
   OPTIONAL_PARALLEL_DIRS := clang
 endif
-
+#ifneq ($(SUBZERO_SRC_ROOT),)

[jvoung (off chromium), 2014/05/22 19:37:58]

Can remove commented out check?

Seems like the point of the variable (and the check for the similar
CLANG_SRC_ROOT variable) is if you want to configure a different directory name.
E.g., configure --with-clang-srcdir=<other>:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20111010/129983.html

Otherwise "OPTIONAL_PARALLEL_DIRS += subzero" could work if you just want to
assume the directory is always called "subzero", and you can remove the variable
too?

[Derek Schuff, 2014/05/22 23:21:56]

Removed.
Yes, SUBZERO_SRC_ROOT does not actually exist in llvm/tools but is external. so
it's an absolute path and not just 'subzero'
The Makefile.config.in bit substitutes llvm/tools/subzero with the real path
passed in to Make.

+OPTIONAL_PARALLEL_DIRS += $(SUBZERO_SRC_ROOT)
 # Build LLDB if present. Note LLDB must be built last as it depends on the
 # wider LLVM infrastructure (including Clang).
 OPTIONAL_DIRS := lldb