diff --git a/Makefile b/Makefile
index 63384e96..af33361a 100644
--- a/Makefile
+++ b/Makefile
@@ -15,7 +15,10 @@
 # Version and build configuration variables
 BUILD_CONFIGURATION ?= debug
 WARNINGS_AS_ERRORS ?= true
-SWIFT_CONFIGURATION := $(if $(filter-out false,$(WARNINGS_AS_ERRORS)),-Xswiftc -warnings-as-errors) -Xswiftc -enable-testing
+# Extra swift build flags, appended last so callers can add to (rather than
+# replace) the configuration above.
+SWIFT_CONFIGURATION_EXTRA ?=
+SWIFT_CONFIGURATION := $(if $(filter-out false,$(WARNINGS_AS_ERRORS)),-Xswiftc -warnings-as-errors) -Xswiftc -enable-testing $(SWIFT_CONFIGURATION_EXTRA)
 # Code-coverage instrumentation, layered onto the shared build stages. Empty for
 # ordinary builds; the coverage-* targets opt in via a target-specific value so
 # only those goals compile instrumented binaries.