use absolute path for BUILDDIR

Summary: public
The relative path was triggering a bug in ocamlbuild where it would try to
create ".." for some reason, which already exists. Hence `make -C infer clean
checkCopyright` would always fail for instance.

Reviewed By: jeremydubreil

Differential Revision: D2596307

fb-gh-sync-id: 0dd8d9f

infer/src/Makefile

@@ -23,7 +23,7 @@ OCAML_INCLUDE_DIR = $(shell ocamlc -where)
 
 ROOT = $(shell cd ../.. && pwd)
 
-BUILDDIR = ../_build-infer
+BUILDDIR = $(ROOT)/infer/_build-infer
 ANNOTDIR = $(ROOT)/infer/src/_build
 BINDIR = $(ROOT)/infer/bin
 SCRIPTDIR = $(ROOT)/scripts