--enable-option-checking help

[Ralf Wildenhues]

configure --help documents --enable-option-checking only if some other
AC_ARG_ENABLE is also used.  However, of course it works also when no
other --enable switch is specially handled by this configure, or when
only AC_ARG_WITH's are used or none of either.

When --enable-option-checking is documented in --help output, it appears
above the generic
  --disable-FEATURE       do not include FEATURE (same as --enable-FEATURE=no)
  --enable-FEATURE[=ARG]  include FEATURE [ARG=yes]

lines; it should appear below.

I realize I may be responsible for introducing these bugs.  :-)

Cheers,
Ralf