diff --git a/usr.bin/make/make.h b/usr.bin/make/make.h
index 0fa7dd00e870..14136464835d 100644
--- a/usr.bin/make/make.h
+++ b/usr.bin/make/make.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: make.h,v 1.264 2021/07/31 09:30:17 rillig Exp $	*/
+/*	$NetBSD: make.h,v 1.265 2021/09/12 09:51:14 rillig Exp $	*/
 
 /*
  * Copyright (c) 1988, 1989, 1990, 1993
@@ -142,18 +142,25 @@
 
 #if __STDC_VERSION__ >= 199901L || defined(lint) || defined(USE_C99_BOOLEAN)
 #include <stdbool.h>
+#elif defined(__bool_true_false_are_defined)
+/*
+ * All files of make must be compiled with the same definition of bool.
+ * Since one of the files includes <stdbool.h>, that means the header is
+ * available on this platform.  Recompile everything with -DUSE_C99_BOOLEAN.
+ */
+#error "<stdbool.h> is included in pre-C99 mode"
+#elif defined(bool) || defined(true) || defined(false)
+/*
+ * In pre-C99 mode, make does not expect that bool is already defined.
+ * You need to ensure that all translation units use the same definition for
+ * bool.
+ */
+#error "bool/true/false is defined in pre-C99 mode"
 #else
-#ifndef bool
-typedef unsigned int Boolean;
-#define bool	Boolean
-#endif
-#ifndef true
+typedef unsigned char bool;
 #define true	1
-#endif
-#ifndef false
 #define false	0
 #endif
-#endif
 
 #include "lst.h"
 #include "enum.h"
diff --git a/usr.bin/make/test-variants.sh b/usr.bin/make/test-variants.sh
index 5668aa2016a0..8477f142bea3 100644
--- a/usr.bin/make/test-variants.sh
+++ b/usr.bin/make/test-variants.sh
@@ -1,5 +1,5 @@
 #! /bin/sh
-# $NetBSD: test-variants.sh,v 1.10 2021/04/03 11:08:40 rillig Exp $
+# $NetBSD: test-variants.sh,v 1.11 2021/09/12 09:51:14 rillig Exp $
 #
 # Build several variants of make and run the tests on them.
 #
@@ -22,7 +22,7 @@ testcase() {
 	env -i PATH="$PATH" USETOOLS="no" "$@" \
 		sh -ce "make -s cleandir" \
 	&& env -i PATH="$PATH" USETOOLS="no" "$@" \
-		sh -ce "make -ks all" \
+		sh -ce "make -ks -j6 dependall" \
 	&& size *.o make \
 	&& env -i PATH="$PATH" USETOOLS="no" MALLOC_OPTIONS="JA" \
 		_MKMSG_TEST=":" "$@" \
@@ -120,10 +120,16 @@ testcase USER_CFLAGS="-O0 -ggdb"
 # Ensure that every inline function is declared as MAKE_ATTR_UNUSED.
 testcase USER_CPPFLAGS="-Dinline="
 
+# Is expected to fail with "<stdbool.h> is included in pre-C99 mode".
 testcase USER_CFLAGS="-std=c90" USER_CPPFLAGS="-Dinline="
 
+# This workaround is necessary on NetBSD 9.99 since main.c includes
+# <sys/sysctl.h>, which includes <stdbool.h> even in pre-C99 mode.
+testcase USER_CFLAGS="-std=c90" USER_CPPFLAGS="-Dinline= -DUSE_C99_BOOLEAN"
+
 #testcase USER_CFLAGS="-std=c90 -pedantic" USER_CPPFLAGS="-Dinline="
 
+# Is expected to fail with "<stdbool.h> is included in pre-C99 mode".
 testcase USER_CFLAGS="-ansi" USER_CPPFLAGS="-Dinline="
 
 # config.h does not allow overriding these features