build system: do not modify user variable CFLAGS

closes: https://github.com/rsyslog/rsyslog/issues/56
mention json-c 0.12 support in ChangeLog (due to json-c security issue)
Merge branch 'v7-stable' into v8-stable
mention important change
prepare for 8.2.1 release
Merge branch 'v8-stable'

Conflicts:
	ChangeLog
	configure.ac