Understand common build variables passed as make variables (#6740)

Summary:
Some common build variables like USE_CLANG and
COMPILE_WITH_UBSAN did not work if specified as make variables, as in
`make USE_CLANG=1 check` etc. rather than (in theory less hygienic)
`USE_CLANG=1 make check`. This patches Makefile to export some commonly
used ones to build_detect_platform so that they work. (I'm skeptical of
a broad `export` in Makefile because it's hard to predict how random
make variables might affect various invoked tools.)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6740

Test Plan: manual / CI

Reviewed By: siying

Differential Revision: D21229011

Pulled By: pdillinger

fbshipit-source-id: b00c69b23eb2a13105bc8d860ce2d1e61ac5a355

Makefile

@@ -204,8 +204,17 @@ LDFLAGS += -lrados
 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)"; export PORTABLE="$(PORTABLE)"; "$(CURDIR)/build_tools/build_detect_platform" "$(CURDIR)/make_config.mk"))
+# Detect what platform we're building on.
+# Export some common variables that might have been passed as Make variables
+# instead of environment variables.
+dummy := $(shell (export ROCKSDB_ROOT="$(CURDIR)"; \
+                  export COMPILE_WITH_ASAN="$(COMPILE_WITH_ASAN)"; \
+                  export COMPILE_WITH_TSAN="$(COMPILE_WITH_TSAN)"; \
+                  export COMPILE_WITH_UBSAN="$(COMPILE_WITH_UBSAN)"; \
+                  export PORTABLE="$(PORTABLE)"; \
+                  export ROCKSDB_NO_FBCODE="$(ROCKSDB_NO_FBCODE)"; \
+                  export USE_CLANG="$(USE_CLANG)"; \
+                  "$(CURDIR)/build_tools/build_detect_platform" "$(CURDIR)/make_config.mk"))
 # this file is generated by the previous line to set build flags and sources
 include make_config.mk
 export JAVAC_ARGS