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