Markdown-ify BUILDING.

Change-Id: Icd3cba6522ce47a4dfe699204982b5b692d3d62e
Reviewed-on: https://boringssl-review.googlesource.com/5811
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/util/all_tests.go b/util/all_tests.go
index ded692e..566c3f7 100644
--- a/util/all_tests.go
+++ b/util/all_tests.go
@@ -201,13 +201,13 @@
 // directory is the top of a BoringSSL checkout.
 func setWorkingDirectory() {
 	for i := 0; i < 64; i++ {
-		if _, err := os.Stat("BUILDING"); err == nil {
+		if _, err := os.Stat("BUILDING.md"); err == nil {
 			return
 		}
 		os.Chdir("..")
 	}
 
-	panic("Couldn't find BUILDING in a parent directory!")
+	panic("Couldn't find BUILDING.md in a parent directory!")
 }
 
 func parseTestConfig(filename string) ([]test, error) {