remove unnecessary space in the help message of configure
fix configure.ac issues (Issue #14)

* autoheader failed because of wrong usage of AH_TEMPLATE.
* AC_CHECK_LIB should be used after AC_PROG_CC.
* AC_CHECK_LIB(iconv, ...) fails on Cygwin because iconv.h isn't
  included. Use AC_LINK_IFELSE instead.