diff --git a/config_host.mk.in b/config_host.mk.in
index 57db3f38250b..8e4c7dfec03b 100644
--- a/config_host.mk.in
+++ b/config_host.mk.in
@@ -62,7 +62,7 @@ export CDR_CFLAGS=$(gb_SPACE)@CDR_CFLAGS@
 export CDR_LIBS=$(gb_SPACE)@CDR_LIBS@
 export CDR_TARBALL=@CDR_TARBALL@
 @x_CFLAGS@ export CFLAGS=@CFLAGS@
-export CHECK_PARALLELISM=@CHECK_PARALLELISM@
+export CHECK_PARALLELISM?=@CHECK_PARALLELISM@
 export CLANGPLUGIN_CPPFLAGS=@CLANGPLUGIN_CPPFLAGS@
 export CLUCENE_CFLAGS=$(gb_SPACE)@CLUCENE_CFLAGS@
 export CLUCENE_LIBS=$(gb_SPACE)@CLUCENE_LIBS@
@@ -211,7 +211,7 @@ export GIO_CFLAGS=$(gb_SPACE)@GIO_CFLAGS@
 export GIO_LIBS=$(gb_SPACE)@GIO_LIBS@
 export GIT_LINK_SRC=@GIT_LINK_SRC@
 export GIT_NEEDED_SUBMODULES=@GIT_NEEDED_SUBMODULES@
-export PARALLELISM=@PARALLELISM@
+export PARALLELISM?=@PARALLELISM@
 export GNOMEVFS_CFLAGS=$(gb_SPACE)@GNOMEVFS_CFLAGS@
 export GNOMEVFS_LIBS=$(gb_SPACE)@GNOMEVFS_LIBS@
 @x_GNUCP@ export GNUCOPY=@GNUCP@