Support building with --disable-maintainer-mode and source != build dir

[Saur2000]

If --disable-maintainer-mode is enabled, then the rules for generating parser.[ch] and lexer.[ch] did nothing. This worked fine if the source and build directories are the same as the pre-generated parser.c and lexer.c files would suffice. However, if the build directory is not the same as the source directory, then the rest of the Make rules expect parser.[ch] and lexer.[ch] to have been created in the build directory if their source files (parser.y and lexer.l) are newer than the target files, which can happen in case the source is fetched using Git.

Avoid the problem by copying the files to the build directory if needed.

[itchyny]

This PR introduces GNU make extensions in our Makefile.am for the first time.

[Saur2000]

This PR introduces GNU make extensions in our Makefile.am for the first time.

According to https://pubs.opengroup.org/onlinepubs/009695399/utilities/make.html, the use of variables such as $(<D) and $(@F) are supported by POSIX Make. And even automake itself adds one (from the generated Makefile):

$(am__depfiles_remade):
        @$(MKDIR_P) $(@D)
        @: >>$@

So unless you are referring to something other than those automatic variables, I believe this should be ok.

[itchyny]

Ah, sorry. I was wrong.

[itchyny]

This fixes #3371, right?

[Saur2000]

This fixes #3371, right?

Yes, it appears that it does. :)

[itchyny]

Thank you.