fix-ltdl: Fix order-only hack with HP-UX make.

HP-UX make gets confused by the space in $(findstring ...,$(...))
syntax: instead of expanding to the empty string as desired, it
mismatches the parentheses and we are left with a single ).  E.g.:

  % cat >Makefile <<'EOF'
  all:
	@echo '$(a $(b))'
EOF
  % make
  )

Since $(x) and ${x} are equivalent in Make, a neat solution is to
just replace one set of parentheses with braces; now there are no
nested parentheses to mismatch in the first place.