Add missing space to separate arguments.
diff --git a/build_script.py b/build_script.py
index 6994f64..8681155 100755
--- a/build_script.py
+++ b/build_script.py
@@ -185,7 +185,7 @@
"-module-name XCTest -module-link-name XCTest -parse-as-library "
"-emit-module-path {build_dir}/XCTest.swiftmodule "
"-force-single-frontend-invocation "
- "-swift-version 3"
+ "-swift-version 3 "
"-I {foundation_build_dir} -I {core_foundation_build_dir} "
"{libdispatch_args} "
"{source_paths} -o {build_dir}/XCTest.o".format(