Add /config.cache to .gitignore in back branches

For some reason config.cache was not being git-ignored in these back
branches.

Backpatch-through: 9.2 to 9.4

Author: bmomjian

.gitignore

@@ -28,6 +28,7 @@ lib*.pc
 
 # Local excludes in root directory
 /GNUmakefile
+/config.cache
 /config.log
 /config.status
 /pgsql.sln