check for and require a C99 capable compiler

recent logging changes use __VA_ARGS__.

configure.ac

@@ -22,6 +22,12 @@ AC_CONFIG_HEADER([config.h])
 AC_PROG_CC([cc gcc])
 AC_CANONICAL_HOST
 AC_C_BIGENDIAN
+AC_PROG_CC_C99
+
+# XXX relax this after reimplementing logit() etc.
+if test "x$ac_cv_prog_cc_c99" = "xno" ; then
+	AC_MSG_ERROR([*** OpenSSH requires a C99 capable compiler ***])
+fi
 
 # Checks for programs.
 AC_PROG_AWK