Use correct target path for Flex check

Signed-off-by: Ralph Castain <rhc@pmix.org>

configure.ac

@@ -627,16 +627,14 @@ AC_PROG_LEX(noyywrap)
 # required for developers, we decided that it really was not worth it
 # to be portable between different versions of lex ;-).
 
-if test -z "$LEX" || \
-   test -n "`echo $LEX | $GREP missing`" || \
-   test "`basename $LEX`" != "flex"; then
-    if test ! -f "$srcdir/src/util/show_help_lex.c"; then
-        AC_MSG_WARN([*** Could not find Flex on your system.])
-        AC_MSG_WARN([*** Flex is required for developer builds of PRTE.])
-        AC_MSG_WARN([*** Other versions of Lex are not supported.])
-        AC_MSG_WARN([*** NOTE: If you are building from a tarball downloaded from the PRTE web site, you do not need Flex.])
-        AC_MSG_ERROR([Cannot continue])
-    fi
+if test "x$LEX" != xflex && test ! -e $PRTE_TOP_SRCDIR/src/util/hostfile/hostfile_lex.c; then
+    AC_MSG_WARN([*** Could not find Flex on your system.])
+    AC_MSG_WARN([*** Flex is required for developer builds of PRRTE.])
+    AC_MSG_WARN([*** Other versions of Lex are not supported.])
+    AC_MSG_WARN([*** NOTE: If you are building from an official tarball])
+    AC_MSG_WARN([*** (not the ones made by GitHub!) downloaded from the])
+    AC_MSG_WARN([*** PRRTE web site, you do not need Flex.])
+    AC_MSG_ERROR([Cannot continue])
 fi
 
 #