This reverts merge request !636

if taken from Git it's also distributed in the installation directory

otherwise fall back on the provided get_host_binary_tag.py

The __init__.py files generated from GaudiProjectConfig.cmake has
code like:

```
__path__ = [d for d in [os.path.join(d, '${pypack}') for d in sys.path if d]
            if (d.startswith('${CMAKE_BINARY_DIR}') or
 ```

However, if the working path contains a symlink, then the element in
sys.path may have the symlink, but the value from ${CMAKE_BINARY_DIR}
to which we compare it will have the symlink expanded.  Change to expand
any symlinks (using os.path.realname) in the paths from sys.path
before comparing.

it's causing troubles and not fixing any real problem (anymore)

gaudi_add_compile_test can be used to add tests that check for the
existence of (expected) compilation failures. Use this to implement a
unit test to check for StatusCode comilation failures.

  - CPP_GSL_INCLUDE_DIR -> CPPGSL_INCLUDE_DIR
  - CPP_GSL_INCLUDE_DIRS -> CPPGSL_INCLUDE_DIRS
  - CPP_GSL_FOUND -> CPPGSL_FOUND
  - RANGES_V3_INCLUDE_DIR -> RANGEV3_INCLUDE_DIR
  - RANGES_V3_INCLUDE_DIRS -> RANGEV3_INCLUDE_DIRS
  - RANGES_V3_FOUND -> RANGEV3_FOUND

I didn't rename all "internal" variables of externals.cmake however, as there didn't
seem to be much point in doing that.

Updated the external lookup code to make use of the new Findcppgsl.cmake and FindRangev3.cmake modules.

if a file a dictionary depends on is removed between builds, the
dictionary is correctly regenerated.

See GAUDI-1315

small changes meant to help the automatic formatting tools take
better decisions.

and updated the supported versions

In CMake 3.9 the OLD behaviour is deprecated, so we must switch to the
new behaviour and to require CMake >= 2.8.12.

See https://cmake.org/cmake/help/v3.9/policy/CMP0026.html

also enable c++1z for clang >= 3.9

by helping other tools like the lcg-g++-x.y.z wrappers used in LHCb.

so that something like *-*-*-opt+g actually works