Maintenance: Drop an unused, misleading condition (#1169)

Maintenance: Drop an unused, misleading condition (#1169)

According to Autoconf NEWS, AC_USE_SYSTEM_EXTENSIONS is defined with
AC_DEFUN_ONCE since Autoconf v2.63b: "This means a subtle change in
semantics; previously, an AC_DEFUN macro could expand [this macro]
multiple times or surround the macro inside shell conditional text to
bypass [its] effects, but now the macro will expand exactly once, and
prior to the start of any enclosing AC_DEFUN macro".

In the generated ./configure script, the if statement no longer had any
code inside it, and AC_USE_SYSTEM_EXTENSIONS was unconditionally
expanded above the if statement.