Revert r27601 - breaks Mac builds as the pattern match doesn't correctly parse the flex --version output

This commit was SVN r27608.

The following SVN revision numbers were found above:
  r27601 --> open-mpi/ompi@c2fa252c14

autogen.pl

@@ -56,14 +56,12 @@ my $exclude_list;
 my $ompi_automake_version = "1.11.1";
 my $ompi_autoconf_version = "2.65";
 my $ompi_libtool_version = "2.2.6b";
-my $ompi_flex_version = "2.5.35";
 my $ompi_m4_version = "1.4.16";
 
 # Search paths
 my $ompi_autoconf_search = "autoconf";
 my $ompi_automake_search = "automake";
 my $ompi_libtoolize_search = "libtoolize;glibtoolize";
-my $ompi_flex_search = "flex";
 my $ompi_m4_search = "m4;gm4";
 
 # One-time setup
@@ -802,7 +800,6 @@ GNU tools:
     GNU Automake: $ompi_automake_version
     GNU Libtool: $ompi_libtool_version
     GNU M4: $ompi_m4_version
-    Flex: $ompi_flex_version
 =================================================================\n";
     my_exit(1);
 }
@@ -1021,7 +1018,6 @@ $step. Checking tool versions\n\n";
 &find_and_check("autoconf", $ompi_autoconf_search, $ompi_autoconf_version);
 &find_and_check("libtool", $ompi_libtoolize_search, $ompi_libtool_version);
 &find_and_check("automake", $ompi_automake_search, $ompi_automake_version);
-&find_and_check("flex", $ompi_flex_search, $ompi_flex_version);
 &find_and_check("m4", $ompi_m4_search, $ompi_m4_version);
 
 #---------------------------------------------------------------------------