Use AC_CONFIG_HEADER as documented by Autoconf manual: call it

only once at the top of the file.  I'm surprised include/config.h wasn't
being overwritten from interfaces/odbc/config.h.in ...

src/configure.in

 dnl Process this file with autoconf to produce a configure script.
 AC_INIT(backend/access/common/heaptuple.c)
 AC_PREFIX_DEFAULT(/usr/local/pgsql)
+AC_CONFIG_HEADER(include/config.h)
 
 AC_CANONICAL_HOST
 
@@ -342,8 +343,6 @@ LDFLAGS="$LDFLAGS $PGSQL_LDFLAGS"
 export LDFLAGS
 echo "- setting LDFLAGS=$LDFLAGS"
 
-AC_CONFIG_HEADER(include/config.h)
-
 dnl Checks for programs.
 AC_PROG_CPP
 
@@ -896,8 +895,6 @@ then
 	fi
 	AC_SUBST(USE_ODBC)
 fi
- 
-AC_CONFIG_HEADER(interfaces/odbc/config.h)
 
 AC_OUTPUT(
 	GNUmakefile