Fix race with LLVM and bison.

Hi All.

I've found build error in configuration --with-llvm

CPPFLAGS="-O2" ./configure --enable-debug --enable-cassert
--enable-tap-tests --with-openssl --with-icu  --with-llvm

....

make world-bin -j3

....

cubescan.c:9:10: fatal error: 'cubeparse.h' file not found
    9 | #include "cubeparse.h"  /* must be after cubedata.h for YYSTYPE
and NDBOX */

....
segscan.c:9:10: fatal error: 'segparse.h' file not found
    9 | #include "segparse.h"   /* must be after segdata.h for SEG */

The reason is race, that exist between LLVM compilation and bison source
code generation and compilation can occur first.

Ideally LLVM compilation target should depend on header files targets.

The error is difficult to reproduce and I've done simple patch to have
stable reproducing. Fix patch is also attached.

Thanks.