bug#54390: YACC rules don't check DESTDIR existence for VPATH builds

[Sam James]

Hi,

It appears that YACC rules don't check for whether the destination directory 
exists before executing ylwrap.

When trying to package libaacs (https://code.videolan.org/videolan/libaacs) 
with an out-of-source build, I hit an unexpected build failure:
```
/var/tmp/portage/media-libs/libaacs-0.11.1/work/libaacs-0.11.1/src/file/keydbcfg-parser.y:
 warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]
/var/tmp/portage/media-libs/libaacs-0.11.1/work/libaacs-0.11.1/build-aux/ylwrap:
 206: cannot create ../src/file/keydbcfg-parser.c: Directory nonexistent
updating src/file/keydbcfg-parser.h
mv: cannot move 'tmp-keydbcfg-parser.h' to '../src/file/keydbcfg-parser.h': No 
such file or directory
make: *** [Makefile:1150: src/file/keydbcfg-parser.c] Error 2
```

I can workaround this by running `mkdir -p ${BUILD_DIR}/src/file` to ensure 
that the necessary directory exists within the build directory beforehand, but
it feels like I shouldn't have to.

Their Makefile.am can be found here: 
https://code.videolan.org/videolan/libaacs/-/blob/master/Makefile.am. Snippet:
```
libaacs_la_SOURCES=\
        src/libaacs/aacs.h \
[...]
        src/file/dirs.h \
        src/file/file.h \
        src/file/file.c \
        src/file/filesystem.h \
        src/file/filesystem.c \
        src/file/keydbcfg.c \
        src/file/keydbcfg.h \
        src/file/keydb.h \
        src/file/keydbcfg-parser.y \
        src/file/keydbcfg-lexer.l \
        src/file/mmc_device.h \
[...]
```

While src/libaacs exists within the build dir, src/file/ doesn't exist at all, 
hence the failure.

automake yacc rules should mkdir -p the needed directories within the build dir 
for VPATH builds before running ylwrap/yacc.

Best,
sam

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