Correct build exclusion in gitignore (#6870)

The build* entry in .gitignore was intended to exclude any build
directories with whatever suffixes a developer might use for various
build configurations. The way it was written made it exclude any file
that had build in the name, which was more aggressive than needed.

By adding a leading slash, it is limited to items in the git root
directory. By adding a trailing slash, it is limited to directories in
the root directory. This accomplishes the intended exclusions as
narrowly as possible.

(I swear I'm not trying to win a bet on writing the longest description
of a two character change in history)

Fixes #4733

.gitignore

@@ -52,7 +52,7 @@ autoconf/autom4te.cache
 # Directories to ignore (do not add trailing '/'s, they skip symlinks).
 #==============================================================================#
 # Build directories
-build*
+/build*/
 # External projects that are tracked independently.
 external/*
 # Clang, which is tracked independently.