update OpenMP support info in build basics

The majority of OpenMP (threading) support in LAMMPS is provided by

the USER-OMP package; see the "Speed omp"_Speed_omp.html doc page for

details.

details. The USER-INTEL package also provides OpenMP support (it is

compatible with USER-OMP) and adds vectorization support when compiled

with the Intel compilers on top of that. Also, the KOKKOS package can

be compiled for using OpenMP threading.

However, there are a few commands in LAMMPS that have native OpenMP

support.  These are commands in the MPIIO, SNAP, USER-COLVARS, and

USER-DPD packages.  See the "Packages details"_Packages_details.html

doc page for more info on these packages and the doc pages for their

respective commands for OpenMP threading info.

TODO: is this the complete list of native OpenMP commands in LAMMPS?

support.  These are commands in the MPIIO, SNAP, USER-DIFFRACTION, and

USER-DPD packages.  In addition some commands support OpenMP threading

not directly, but through the libraries they are interfacing to:

e.g. LATTE and USER-COLVARS. See the "Packages

details"_Packages_details.html doc page for more info on these packages

and the doc pages for their respective commands for OpenMP threading

info.

For CMake, if you use BUILD_OMP=yes, then you can use these packages

and turn on their native OpenMP support at run time, by first setting

the OMP_NUM_THREADS environment variable.

For make, ...

TODO: how do we build LAMMPS with make, to include OpenMP support

(separate from USER-OMP package).  Akin to CMake with BUILD_OMP=yes.

For the conventional makefiles, the CCFLAGS and LINKFLAGS variables

need to include the compiler flag, that enables OpenMP. For GNU

compilers, this flag is -fopenmp, for (recent) Intel compilers,

it is -qopenmp. Please refer to the documentation of your compiler,

if you are using a different compiler to compile LAMMPS.

:line