* [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`:
cb6b311c39/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.
d91ec8500f