mu4e: use mu4e-config instead of mu4e-meta

All these changes to avoid:

,----
| mu4e/meson.build:92: WARNING: Source item '/home/djcb/Sources/mu/build/mu4e/mu4e-meta.el' cannot be converted to File object, because it is a generated file. This will become a hard error in the future
`----

This is because we want to byte-compile a file we just before generated using
configure_file. This does not seem like a crazy thing, but meson threatens with
breaking the build at some point in the future.

So instead, we decide _not_ to compile this (very boring) file. But, users may
still have an older mu4e-meta.elc lying around, leading to confusion.

So, let's rename that file and we're golden.

.gitignore

@@ -89,7 +89,7 @@ test-sexp-parser
 test-scanner
 /guile/tests/test-mu-guile
 
-mu4e-meta.el
+mu4e-config.el
 mu4e.pdf
 texinfo.tex
 texi.texi