build: make it easier to pass PORTABLE

Summary:
currently when running a portable build we have to do the following

  PORTABLE=1 make ...

this commit adds support for the following

  make PORTABLE=1 ...

this might be seem subtle but it makes PORTABLE like all other
makefile args and simplifies invocation from numerous build systems
including things like ExternalProject_Add in cmake.

Signed-off-by: Bassam Tabbara <bassam.tabbara@quantum.com>
Closes https://github.com/facebook/rocksdb/pull/1643

Differential Revision: D4315870

Pulled By: yiwu-arbug

fbshipit-source-id: ee43755

Makefile

@@ -140,7 +140,7 @@ endif
 
 AM_LINK = $(AM_V_CCLD)$(CXX) $^ $(EXEC_LDFLAGS) -o $@ $(LDFLAGS) $(COVERAGEFLAGS)
 # detect what platform we're building on
-dummy := $(shell (export ROCKSDB_ROOT="$(CURDIR)"; "$(CURDIR)/build_tools/build_detect_platform" "$(CURDIR)/make_config.mk"))
+dummy := $(shell (export ROCKSDB_ROOT="$(CURDIR)"; export PORTABLE="$(PORTABLE)"; "$(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
 CLEAN_FILES += make_config.mk