diff --git a/t/test-lib.sh b/t/test-lib.sh
index fe688a3fce..62dfcc4aaf 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -35,7 +35,7 @@ else
 	# needing to exist.
 	TEST_DIRECTORY=$(cd "$TEST_DIRECTORY" && pwd) || exit 1
 fi
-GIT_BUILD_DIR="${TEST_DIRECTORY%/t}"
+GIT_BUILD_DIR="${GIT_BUILD_DIR:-${TEST_DIRECTORY%/t}}"
 if test "$TEST_DIRECTORY" = "$GIT_BUILD_DIR"
 then
 	echo "PANIC: Running in a $TEST_DIRECTORY that doesn't end in '/t'?" >&2
@@ -522,6 +522,7 @@ unset VISUAL EMAIL LANGUAGE $("$PERL_PATH" -e '
 		PERF_
 		CURL_VERBOSE
 		TRACE_CURL
+		BUILD_DIR
 	));
 	my @vars = grep(/^GIT_/ && !/^GIT_($ok)/o, @env);
 	print join("\n", @vars);