Support multi threading on linux

See merge request !39

a build directory. Project Makefiles can set a BUILDDIR variable to their
build directory - this will be used in the generated target names

Mnt/fsldevdir

See merge request !38

RF: Also need to drop -L flags that are set in environment, in addition to -rpath flags

See merge request !37

to -rpath flags

RF: Work around issue with c++17 and older versions of libxml++.

See merge request !36

define here rather than in every project which uses ciftilib (which in turn
uses libxml++)

Bf/cudacxxflags

See merge request !35

RF: New CUDACXXFLAGS

See merge request !34

into a CUDA lib / exe - so that USRNVCCFLAGS, and CUDA include paths are added
to compiler flags for these targets.

RF: Set -std=c++14 for nvcc-compiled code - CUDA 11.0 is the earliest to support c++17

See merge request !33

support c++17

Mnt/c++17

See merge request !32

CI: Don't run CI tests on tags or master branch

See merge request !31

Mnt/fsldevdir rpath

See merge request !30

Mnt/post 606

See merge request !29