Support building with --disable-maintainer-mode and source != build dir (#3518)

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.

Co-authored-by: Peter Kjellerstedt <pkj@axis.com>