make help: Suggest using -j1 -k options with make syntax-check.

This lets you see all the output, not interleaved.
Ignore various intermediate files left around by 'make syntax-check'.
syntax-check: Ignore more syntax-check problems.

For some reason maint.mk doesn't process all of these regexps,
or perhaps the regexps are not well-formed.
syntax-check: Use exit (EXIT_*) macros instead of hard-coded 0|1.

These are defined in <stdlib.h> which the test program already includes.
syntax-check: Remove unused ``#includes''.
syntax-check: Use newline before EOF in these text files.
syntax-check: Remove trailing whitespace.
syntax-check: HAVE_DECL_* macros are always defined.

These macros are always defined to 0|1.  We don't need to
check if they are defined.