diff --git a/share/mk/bsd.own.mk b/share/mk/bsd.own.mk
index 1ef2dd50a41..3565425086f 100644
--- a/share/mk/bsd.own.mk
+++ b/share/mk/bsd.own.mk
@@ -268,6 +268,7 @@ __DEFAULT_YES_OPTIONS = \
     ED_CRYPTO \
     EXAMPLES \
     FLOPPY \
+    FORMAT_EXTENSIONS \
     FORTH \
     FP_LIBC \
     FREEBSD_UPDATE \
diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
index e42b753e607..99211f0f4bf 100644
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -5,7 +5,7 @@
 #
 CWARNFLAGS?=	-Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes \
 		-Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual \
-		-Wundef -Wno-pointer-sign -fformat-extensions \
+		-Wundef -Wno-pointer-sign ${FORMAT_EXTENTIONS} \
 		-Wmissing-include-dirs -fdiagnostics-show-option \
 		${CWARNEXTRA}
 #
@@ -29,7 +29,15 @@ NO_WSOMETIMES_UNINITIALIZED=	-Wno-error-sometimes-uninitialized
 # enough to error out the whole kernel build.  Display them anyway, so there is
 # some incentive to fix them eventually.
 CWARNEXTRA?=	-Wno-error-tautological-compare -Wno-error-empty-body \
-		-Wno-error-parentheses-equality
+		-Wno-error-parentheses-equality ${NO_WFORMAT}
+.endif
+
+# External compilers may not support our format extensions.  Allow them
+# to be disabled.  WARNING: format checking is disabled in this case.
+.if ${MK_FORMAT_EXTENSIONS} == "no"
+NO_WFORMAT=		-Wno-format
+.else
+FORMAT_EXTENTIONS=	-fformat-extensions
 .endif
 
 #
diff --git a/tools/build/options/WITHOUT_FORMAT_EXTENSIONS b/tools/build/options/WITHOUT_FORMAT_EXTENSIONS
new file mode 100644
index 00000000000..7ae02e1bf86
--- /dev/null
+++ b/tools/build/options/WITHOUT_FORMAT_EXTENSIONS
@@ -0,0 +1,5 @@
+.\" $FreeBSD$
+Set to not enable
+.Fl fformat-extensions
+when compiling the kernel.
+Also disables all format checking.