Work around stdbool problem in dfmgr.c.

Work around stdbool problem in dfmgr.c.

Commit 842cb9fa6 refactored things so that dfmgr.c includes <dlfcn.h>,
which before that had only been directly included in platform-specific
stub files.  It turns out that on macOS, <dlfcn.h> includes <stdbool.h>,
and that causes problems on platforms where _Bool is not char-sized ...
which happens to include the PPC versions of macOS.  Work around it
much as we have in plperl.h, by #undef'ing bool after including the
problematic file, but only if we're not using stdbool-style booleans.

Discussion: https://postgr.es/m/E1fxqjl-0003YS-NS@gemulon.postgresql.org