Fix three Flymake bugs when checking C header files

The first of these problems is longstanding: if an error-less B.h is
included from error-ridden A.h, flymake's legacy parser will panic
(and disable itself) since it sees a non-zero exit for a clean file.
To fix this, recommend returning 'true' in the documentation for the
check-syntax target.

Another problem was introduced by the parser rewrite.  For error
patterns spanning more than one line, point may be left in the middle
of a line and thus render other patterns useless.  Those patterns were
written for the old line-by-line parser.  To make them useful again,
move to the beginning of line in those situations.

The third problem was also longstanding and happened on newer GCC's:
The "In file included from" prefix confused
flymake-proc-get-real-file-name.  Fix this.

Also updated flymake--diag-region to fallback to highlighting a full
line less often.

Add automatic tests to check this.

* lisp/progmodes/flymake-proc.el
(flymake-proc--diagnostics-for-pattern): Fix bug when patterns
accidentally spans more than one line.  Don't create
diagnostics without error messages.
(flymake-proc-real-file-name-considering-includes): New
helper.
(flymake-proc-allowed-file-name-masks): Use it.

* lisp/progmodes/flymake.el (flymake-diag-region): Make COL
argument explicitly optional.  Only fall back to full line in extreme
cases.

* test/lisp/progmodes/flymake-tests.el
(included-c-header-files): New test.
(different-diagnostic-types): Update.

* test/lisp/progmodes/flymake-resources/Makefile
(check-syntax): Always return success (0) error code.
(CC_OPTS): Add -Wextra

* test/lisp/progmodes/flymake-resources/errors-and-warnings.c
(main): Rewrite comments.

* test/lisp/progmodes/flymake-resources/errors-and-warnings.c:
Include some dummy header files.

* test/lisp/progmodes/flymake-resources/no-problems.h: New file.

* test/lisp/progmodes/flymake-resources/some-problems.h: New file.

* doc/misc/flymake.texi (Example---Configuring a tool called
via make): Recommend adding "|| true" to the check-syntax target.

1 files changed, 9 insertions, 6 deletions

diff --git a/test/lisp/progmodes/flymake-resources/errors-and-warnings.c b/test/lisp/progmodes/flymake-resources/errors-and-warnings.c
index 6454dd20236..1d38bd6bd27 100644
--- a/test/lisp/progmodes/flymake-resources/errors-and-warnings.c
+++ b/test/lisp/progmodes/flymake-resources/errors-and-warnings.c
@@ -1,10 +1,13 @@
- int main()
+/* Flymake should notice an error on the next line, since
+   that file has at least one warning.*/
+#include "some-problems.h"
+/* But not this one */
+#include "no-problems.h"
+
+int main()
 {
-  char c = 1000;
+  char c = 1000; /* a note and a warning */
   int bla;
-  /* The following line should have one warning and one error. The
-     warning spans the full line because gcc (at least 6.3.0) points
-     places the error at the =, which isn't a sexp.*/
-  char c; if (bla == (void*)3);
+  char c; if (bla == (void*)3); /* an error, and two warnings */
   return c;
 }