Update to 2.18.3.

• Be less strict in the check for MASTER_SITE_SUBDIR use. This change checks that the subdirectory used is actually different than what is defined in MASTER_SITES_SUBDIRS. To make this easier to do, support has been added to single-quote variables passed to the get_makevar* functions. [1]

• Now only PY_FLAVOR is recommended as opposed to FLAVOR. [2]

• Remove an errant ':' when suggesting PY_FLAVOR. [3]

PR: 228227 [1]
Requested by: mat [2]
Reported by: koobs [3]