diff --git a/Source/cmListFileLexer.c b/Source/cmListFileLexer.c
index 8c7e0664a..9c4f4dfb7 100644
--- a/Source/cmListFileLexer.c
+++ b/Source/cmListFileLexer.c
@@ -484,6 +484,7 @@ static yyconst flex_int32_t yy_rule_can_match_eol[15] =
 /* Disable features we do not need. */
 #define YY_NEVER_INTERACTIVE 1
 #define YY_NO_UNPUT 1
+#define YY_NO_UNISTD_H 1
 #define ECHO
 
 /* Setup the proper yylex declaration.  */
diff --git a/Source/cmListFileLexer.l b/Source/cmListFileLexer.l
index 9f272e0da..7878344b0 100644
--- a/Source/cmListFileLexer.l
+++ b/Source/cmListFileLexer.l
@@ -34,6 +34,7 @@ Modify cmListFileLexer.c:
 /* Disable features we do not need. */
 #define YY_NEVER_INTERACTIVE 1
 #define YY_NO_UNPUT 1
+#define YY_NO_UNISTD_H 1
 #define ECHO
 
 /* Setup the proper yylex declaration.  */