cdecl99: Don't check blank lines if no history support.

cdecl99: Don't check blank lines if no history support.

Move the whole "if not blank, then add history" logic under the #ifdef,
so it can be eliminated completely if add_history is not available at
compile time.

This probably makes no major difference on GNU/Linux, since GCC knows
that strspn has no side effects and can drop the call as dead code, but
this may help with less sophisticated toolchains.