diff --git a/lib/asn1/NTMakefile b/lib/asn1/NTMakefile
index 79b6b5d41..210b7dfa2 100644
--- a/lib/asn1/NTMakefile
+++ b/lib/asn1/NTMakefile
@@ -519,6 +519,9 @@ $(BINDIR)\asn1_compile.exe:				    \
 $(OBJ)\lex.c: lex.l $(OBJ)\asn1parse.h
 	$(LEX) -o$@ lex.l
 
+$(OBJ)\lex.obj: $(OBJ)\lex.c
+	$(C2OBJ) -DYY_NO_UNISTD_H
+
 $(OBJ)\asn1parse.c $(OBJ)\asn1parse.h: asn1parse.y
 	$(YACC) -o $(OBJ)\asn1parse.c --defines=$(OBJ)\asn1parse.h $**
 
diff --git a/lib/com_err/NTMakefile b/lib/com_err/NTMakefile
index 3bf15bf2c..531b8ab41 100644
--- a/lib/com_err/NTMakefile
+++ b/lib/com_err/NTMakefile
@@ -66,7 +66,7 @@ $(OBJ)\parse.obj: $(OBJ)\parse.c
 	$(C2OBJ) -I$(SRC)\$(RELDIR)
 
 $(OBJ)\lex.obj: $(OBJ)\lex.c
-	$(C2OBJ) -I$(SRC)\$(RELDIR)
+	$(C2OBJ) -I$(SRC)\$(RELDIR) -DYY_NO_UNISTD_H
 
 $(OBJ)\compile_et.obj: compile_et.c
 	$(C2OBJ) -I$(OBJ)
diff --git a/lib/hx509/NTMakefile b/lib/hx509/NTMakefile
index e7b73d72a..24d696cc0 100644
--- a/lib/hx509/NTMakefile
+++ b/lib/hx509/NTMakefile
@@ -218,7 +218,7 @@ $(OBJ)\sel-gram.obj: $(OBJ)\sel-gram.c
 	$(C2OBJ) -I$(SRCDIR)
 
 $(OBJ)\sel-lex.obj: $(OBJ)\sel-lex.c
-	$(C2OBJ) -I$(SRCDIR) -I$(OBJ)
+	$(C2OBJ) -I$(SRCDIR) -I$(OBJ) -DYY_NO_UNISTD_H
 
 $(OBJ)\sel-gram.c: sel-gram.y
 	$(YACC) -o $@ --defines=$(OBJ)\sel-gram.h sel-gram.y
diff --git a/lib/sl/NTMakefile b/lib/sl/NTMakefile
index c56da23a7..285c5d7f1 100644
--- a/lib/sl/NTMakefile
+++ b/lib/sl/NTMakefile
@@ -52,7 +52,7 @@ $(OBJ)\slc-lex.c: slc-lex.l
 	$(LEX) -o$@ slc-lex.l
 
 $(OBJ)\slc-lex.obj: $(OBJ)\slc-lex.c
-	$(C2OBJ) -I$(SRC)\$(RELDIR)
+	$(C2OBJ) -I$(SRC)\$(RELDIR) -DYY_NO_UNISTD_H
 
 $(OBJ)\test_sl.exe: $(OBJ)\test_sl.obj
 	$(EXECONLINK) $(LIBSL) $(LIBVERS) $(LIBROKEN)