[build] Tighten .gitignore exclude rules to ignore dot files in /src/…

…*/ (#2004)

.gitignore

@@ -1,7 +1,7 @@
 # Compiled extensionless executable files in /src/*/
 # This stanza must precede wildcard patterns below!
 /src/*/*
-!/src/*/*.*
+!/src/*/?*.*
 !/src/doc/*
 !/src/*/Makefile
 !/src/*/README