Add flycheck-pos-tip extension

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Add flycheck-rust extension

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[Fix #434] Expand default-directory before using it
Ensure C++ checkers actually instantiate templates.

When using -fsyntax-only, GCC checks mostly for syntax errors, doing
only quite basic analysis. It won't attempt to instantiate templates.
This is unlike Clang, which reports all warnings and errors, including
the errors, for example, deep in the bowels of some template code.

This changeset ensures both compilers will show errors on ill-formed
code, even in presence of templates.
Unbreak the GCC -fno-rtti test case.
Merge pull request #459 from Fanael/master

Ensure C++ checkers actually instantiate templates.
Update changelog [#459]
Remove unnecessary temporary buffers
Add null-device symbol for syntax checker commands
Use null-device instead of temporary-file-name
Merge branch 'null-device'

* null-device:
  Use null-device instead of temporary-file-name
  Add null-device symbol for syntax checker commands
Update Changelog and Texinfo manual