CFLAGS is a user variable, extracted from the environment at configure time
and settable by the user at build time. We must not override this variable.
Get rid of almost all uses of these definitions. They're still defined for
delinquent out-of-tree drivers, and also for the Mesa build. As well as
for miinitext.c. But largely gone.
Dist helper shell scripts.
Dist XAA.HOWTO and a few more unused C files.
Dist xorgconf.cpp.
Fix DIST_EXTRA typo.
Clean yacc and lex generated files only during make maintainer-clean as we
don't expect users to have those tools installed.