[gromacs] Fix intel (classic) libstdc++ path (#37822)

* [gromacs] Fix intel (classic) libstdc++ path

Gromacs's `cmake` run will look for `--gcc-toolchain` (e.g. LLVM, icpx) or
`--gcc-name` (e.g. icpc) in `CMAKE_CXX_FLAGS`. Only if it does not find a good
g++ candidate there it will look for `GMX_GPLUSPLUS_PATH`:
https://github.com/gromacs/gromacs/blob/cb6b311c39fc726a72170c4593b754c8d0a492ac/cmake/FindLibStdCpp.cmake#L97

Spack installed intel compilers already define a g++ for std libs. But in
`icp{c,x}.cfg` instead of the compile line. If we use the pre-defined g++ we not
only have less chance of mixing g++ versions, but also don't need to explicitly
add `gcc` as dependency to `gromacs`.

* Fix format

* Use a variant

As there is no way to check if a file exists at depends_on stage

* Fix format

* New name and fail if variant is used with other compiler

* Line too long.

0 files changed, 0 insertions, 0 deletions