Only check for python location once (#7123)

Summary:
This fixes an issue introduced in 0c56fc4 whereby the location of Python is evaluated many times and leads to excessive logging of unknown python locations of CentOS 6.

The location is now only checked once.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7123

Reviewed By: zhichao-cao

Differential Revision: D22532274

Pulled By: ajkr

fbshipit-source-id: cade71b4b46e9a23d63ecb4dd36a4ac8ae217970

Makefile

@@ -9,7 +9,9 @@
 BASH_EXISTS := $(shell which bash)
 SHELL := $(shell which bash)
 # Default to python3. Some distros like CentOS 8 do not have `python`.
-PYTHON?=$(shell which python3 || which python || echo python3)
+ifeq ($(origin PYTHON), undefined)
+	PYTHON := $(shell which python3 || which python || echo python3)
+endif
 export PYTHON
 
 CLEAN_FILES = # deliberately empty, so we can append below.