build: fix unportable Makefile syntax

Summary:
* Makefile (dummy): Prefix this statement with "dummy := ",
so that it no longer triggers a syntax error from GNU make 3.80
and earlier.  Reported by nielsl in
https://github.com/facebook/rocksdb/issues/509

Test Plan: run make

Reviewers: sdong, ljin, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D33429

Makefile

@@ -31,7 +31,7 @@ endif
 #-----------------------------------------------
 
 # detect what platform we're building on
-$(shell (export ROCKSDB_ROOT="$(CURDIR)"; "$(CURDIR)/build_tools/build_detect_platform" "$(CURDIR)/make_config.mk"))
+dummy := $(shell (export ROCKSDB_ROOT="$(CURDIR)"; "$(CURDIR)/build_tools/build_detect_platform" "$(CURDIR)/make_config.mk"))
 # this file is generated by the previous line to set build flags and sources
 include make_config.mk