- downgrades some diagnostics about nonconformant code from errors to
  warnings. Oddly enough, the errors actually arise from STL library
  elements shipped with gcc itself. Affects kahip compilation with
  gcc-6, gcc-7

- required if there is no system openmp and libomp or libgomp are
  only found in the clang hierarchy

STYLE: add some notes in the openmp rules.

- the _OPENMP macro is now used in low-level testing files

- initial settings for ADIOS2, linuxARM64Gcc, linuxARM64Clang

- the USE_OMP preprocessor symbol is also defined with the openmp
  compile flag to allow conditional compilation of openmp-specific
  code.

The change from C++0x to C++11 allows all of C++11 functionality to be
used in OpenFOAM, in particular constructor delegation which avoids code
duplication or constructor helper functions.  However, this also means a
change to the minimum gcc version supported which is now 4.7 rather than
4.5.

Note that gcc-4.7 does not support the entire C++11 standard but does
support all of the functionality currently needed for further OpenFOAM
development.  The minimum gcc-version which supports the entire C++11
standard is 4.8 which is now the recommended minimum gcc version.

which may be optionally overridden by version-specific rules.

For example the default rules for gcc on GNU/Linux x86_64 are in the
wmake/rules/linux64Gcc directory.  If there is a need to change any of
the rules for a specific version of gcc, e.g. gcc-4.8.4 the directory
wmake/rules/linux64Gcc48 may be created into which any of the language
files may be provided containing the rules to override the defaults.

The c++0x is used rather than c++11 to support gcc-4.5.?

Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1846

Patch provided by Bruno Santos
Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1846

Also removed __GNUC__ conditional compilation statements which are no
longer needed.

Resolves feature request http://www.openfoam.org/mantisbt/view.php?id=1759