[m-rev.] diff: avoid a configure abort with MSVC

[Julien Fischer]

Avoid a configure abort with MSVC.

configure.ac:
     The AC_PROG_LEX macro we use to detect flex does not work with MSVC.
     Avoid its use if we are using MSVC.

Julien.

diff --git a/configure.ac b/configure.ac
index f62355f..57ac49c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5158,7 +5158,11 @@ AC_SUBST(HWLOC_STATIC_LIBS)
  # would have to flex and bison available in order to install the binary
  # distribution.
  #
-if test "$reconfiguring" = "no"
+# XXX the AC_PROG_LEX macro does not work with MSVC, so we omit the check
+# for flex if using that (as of autoconf 2.71). For MSVC, any problem with
+# the flex program will be caught when attempting to build the trace directory.
+#
+if test "$reconfiguring" = "no" -a "$USING_MICROSOFT_CL_COMPILER" = "no"
  then
      # XXX when we drop support for autoconf < 2.70 we should use the following:
      # AC_PROG_LEX([noyywrap])
@@ -5167,7 +5171,10 @@ then
      then
          AC_MSG_ERROR([You need flex to build Mercury])
      fi
+fi

+if test "$reconfiguring" = "no"
+then
      AC_PROG_YACC
      # AC_PROG_YACC may bind YACC to e.g. "bison -y"; we care only about
      # the program name.