If the spamassassin and sa-learn programs cannot be found with
AC_PATH_PROG, configure will abort with an error message.
You can then either:
a) install the SpamAssassin software
b) specify the path with SPAMASSASSIN=/path/to/spamassassin and/or
SA_LEARN=/path/to/sa-learn
c) pass --disable-spamassassin to configure to exclude the module
This also drops the "spamc" and "spamd" GSettings keys.
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