* arts-2-1-529

* Having fun with compiler bugs.

* src/rte.cc (get_ppath_trans):

The OpenMP loops in this function fail randomly with GCC on Mac OS X.
This appears to be a compiler bug. The code looks fine and they work
fine with GCC on Linux and with the Intel Compiler on Mac.
Also tested to remove the OpenMP statements from our loops and instead
add a dummy OpenMP loop that does nothing. This still leads to crashes
which indicates there's nothing wrong with our loops but with the
compiler implementation. Apple GCC 4.2 works fine but self-compiled
GCC 4.4, 4.6, 4.7 and 4.8dev all experiences this issue.
Added #if !(__APPLE__) || (__INTEL_COMPILER) for those loops.

* Fix compilation with the Intel Compiler (tested with 12.1.0, 13.0.1):

* src/m_xml.{h.cc}:

Move non-template implementations into new .cc file.

* src/absorption.cc:

Always make species_data visible as const.

* src/absorption.cc, src/m_abs_lookup.cc, src/m_batch.cc,
src/m_doit.cc, src/m_rte.cc:

Workaround for Intel Compiler (13.0.1) OpenMP issue. Loops that use
firstprivate variables segfault if the number of iterations is 0. The
Intel Compiler correctly skips the loop in that case, but still tries
to call the destructors of the variables defined as firstprivate which
haven't even been created in this scenario.

* src/rte.cc (get_ppath_abs):

Workaround for Intel Compiler (13.0.1) optimization bug. Remove the
continue statement from within the catch block. Otherwise compilation
will never finish.

* src/m_abs.cc, src/m_ppath.cc: Include m_xml.h for WriteXML.

* src/xml_io_compound_types.cc (xml_read_from_stream<SpeciesAuxData>):

Initialize n to 0 to avoid compiler warning.

* src/CMakeLists.txt:

Use set_source_files_properties for all compilers.

Add m_xml.cc to artscore.

* CMakeLists.txt:

Add compiler flags -fvisibility=hidden -fvisibility-inlines-hidden.

git-svn-id: https://www.sat.ltu.se/svn/rt/arts/trunk@8254 aaf1aab0-4228-0410-ad68-8dceda47f409
* arts-2-1-530

* README: Add info on how to compile with LLVM/Clang.

git-svn-id: https://www.sat.ltu.se/svn/rt/arts/trunk@8258 aaf1aab0-4228-0410-ad68-8dceda47f409