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, 2 insertions, 2 deletions

diff --git a/test/lisp/progmodes/flymake-resources/Makefile b/test/lisp/progmodes/flymake-resources/Makefile
index 0f3f39791c8..494407567f2 100644
--- a/test/lisp/progmodes/flymake-resources/Makefile
+++ b/test/lisp/progmodes/flymake-resources/Makefile
@@ -1,6 +1,6 @@
 # Makefile for flymake tests
 
-CC_OPTS = -Wall
+CC_OPTS = -Wall -Wextra
 
 ## Recent gcc (e.g. 4.8.2 on RHEL7) can automatically colorize their output,
 ## which can confuse flymake.  Set GCC_COLORS to disable that.
@@ -8,6 +8,6 @@ CC_OPTS = -Wall
 ## normally use flymake, so it seems like just avoiding the issue
 ## in this test is fine.  Set flymake-log-level to 3 to investigate.
 check-syntax:
-	GCC_COLORS= $(CC) $(CC_OPTS) ${CHK_SOURCES}
+	GCC_COLORS= $(CC) $(CC_OPTS) ${CHK_SOURCES} || true
 
 # eof