Allow user to name/tag executable

[David Dickinson]

Whilst we can ask GS2 for the version and config, which gives us some information about the origin of the executable, it might be nice to allow a user specified string to be passed at compile time. When experimenting with code changes, doing comparisons etc. one may end up with several builds. Having a user-defined string that we can get from interrogating the executable might help keep track of things.

[David Dickinson]

So something like the following would be possible

make -j gs2 BUILD_TAG='Test of implementation 1'

‌

./gs2 --build-config

Built with GCC version 10.2.0 20200723 (Cray Inc.)
Build flags: ...

Current build configuration:
BUILD_TAG: 'Test of implementation 1'
MPI: T
MPI3: F
FFTW: T
iso_c_binding: T
New diagnostics: T
GENQUAD: F
NetCDF: T
LAPACK: T
Lowflow: F
Eigensolver: F
HDF5: F
F200X intrinsics: T
Single precision: F
Quad precision: F

‌

[Peter Hill]

Add user-settable build tag that is displayed in standard header

Resolves #201

→ <<cset 507b4337af55>>