AC_PROG_CC
AC_PROG_LIBTOOL
-AC_SEARCH_LIBS(__gmpz_init, gmp, , [AC_MSG_ERROR([GNU MP not found, see http://gmplib.org/])])
+AC_ARG_WITH(openssl, AS_HELP_STRING([--with-openssl], [Use OpenSSL instead of GMP (beware of the OpenSSL license)]), [with_openssl=$withval], [with_openssl=no])
+AM_CONDITIONAL(WITH_OPENSSL, [test x"$with_openssl" != xno])
+
+AS_IF([test x"$with_openssl" != xno],
+ [AC_SEARCH_LIBS(BN_init, crypto, ,
+ [AC_MSG_ERROR([OpenSSL not found, see http://www.openssl.org/, or try --without-openssl])])],
+ [AC_SEARCH_LIBS(__gmpz_init, gmp, ,
+ [AC_MSG_ERROR([GNU MP not found, see http://gmplib.org/, or try --with-openssl])])])
AC_ARG_ENABLE(rijndael, AS_HELP_STRING([--disable-rijndael], [Disable build of the d0_rijndael library]), [enable_aes=$enableval], [enable_aes=yes])
AM_CONDITIONAL(ENABLE_RIJNDAEL, [test x$enable_aes = xyes])