fix host-mksh build on nōn-Linux systems

Signed-off-by: Thorsten Glaser <tg@mirbsd.org>

package/mksh/Makefile

@@ -31,7 +31,7 @@ BUILD_STYLE:=		manual
 INSTALL_STYLE:=		manual
 
 host-build:
-	cd ${WRKBUILD} && TARGET_OS=Linux \
+	cd ${WRKBUILD} && \
 	    HAVE_CAN_FSTACKPROTECTORALL=0 HAVE_CAN_FSTACKPROTECTORSTRONG=0 \
 	    ${BASH} ${WRKSRC}/Build.sh -Q -r -c lto