build-sys: always create the output directory first

$(MKDIR_P) is added where missing, and rules are standardized on one
form of $(MKDIR_P), to make it easier to spot when it is missing.
Single line $(MKDIR)&&command form is broken into two line form.

https://bugs.freedesktop.org/show_bug.cgi?id=49459

For compilation in a separate build directory to work, when a file is
generated, the rule must include an explicit mkdir first, unless the
file is created at the top level. Even when building in a separate
build-dir, automake would normally create all directories as a side
result of creating the dependencies files. Therefore the bug was only
visible with -C (turning off dependency generation).

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