Make cppcheck useable again

This adds a few suppression rules for files that we don't change
(3rd-party, auto-generated filter language parser, system-wide headers).

It also prevents those files from participating in cppcheck's
"configurations", which is essentially a set of macros that are defined
simultaneously. By cutting down the number of macros, I improved the
runtime of `make CPPCHECK_JOBS=9 cppcheck` from 41 minutes to 47
*seconds*.

Some of the test files can't be analysed because Catch's macros are too
opaque for cppcheck to handle. I didn't suppress the whole "tests"
directory because there are some seemingly useful warnings in there.

.gitignore

@@ -30,3 +30,4 @@ doc/podboat-cmds-linked.asciidoc
 /target/
 **/*.rs.bk
 .vscode
+/cppcheck.log

Makefile

@@ -283,9 +283,11 @@ fmt:
 
 cppcheck:
 	cppcheck -j$(CPPCHECK_JOBS) --force --enable=all --suppress=unusedFunction \
+		--config-exclude=3rd-party --config-exclude=$(relative_cargo_target_dir) --config-exclude=/usr/include \
+		--suppress=*:3rd-party/* --suppress=*:$(relative_cargo_target_dir)/* --suppress=*:/usr/include/* \
 		-DDEBUG=1 \
 		$(INCLUDES) $(DEFINES) \
-		include filter newsboat.cpp podboat.cpp rss src stfl test \
+		include newsboat.cpp podboat.cpp rss src stfl test \
 		2>cppcheck.log
 	@echo "Done! See cppcheck.log for details."