[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`:
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.

var/spack/repos/builtin/packages/gromacs/package.py

@@ -232,6 +232,13 @@ class Gromacs(CMakePackage, CudaPackage):
         for gmx_ver, plumed_vers in plumed_patches.items():
             depends_on("plumed@{0}".format(plumed_vers), when="@{0}+plumed".format(gmx_ver))
 
+    variant(
+        "intel_provided_gcc",
+        default=False,
+        description="Use this if Intel compiler is installed through spack."
+        + "The g++ location is written to icp{c,x}.cfg",
+    )
+
     depends_on("fftw-api@3")
     depends_on("cmake@2.8.8:3", type="build")
     depends_on("cmake@3.4.3:3", type="build", when="@2018:")
@@ -244,7 +251,8 @@ class Gromacs(CMakePackage, CudaPackage):
     depends_on("sycl", when="+sycl")
     depends_on("lapack", when="+lapack")
     depends_on("blas", when="+blas")
-    depends_on("gcc", when="%oneapi")
+    depends_on("gcc", when="%oneapi ~intel_provided_gcc")
+    depends_on("gcc", when="%intel ~intel_provided_gcc")
 
     depends_on("hwloc@1.0:1", when="+hwloc@2016:2018")
     depends_on("hwloc", when="+hwloc@2019:")
@@ -254,6 +262,14 @@ class Gromacs(CMakePackage, CudaPackage):
 
     depends_on("nvhpc", when="+cufftmp")
 
+    requires(
+        "%intel",
+        "%oneapi",
+        policy="one_of",
+        when="+intel_provided_gcc",
+        msg="Only attempt to find gcc libs for Intel compiler if Intel compiler is used.",
+    )
+
     patch("gmxDetectCpu-cmake-3.14.patch", when="@2018:2019.3^cmake@3.14.0:")
     patch("gmxDetectSimd-cmake-3.14.patch", when="@5.0:2017^cmake@3.14.0:")
 
@@ -431,7 +447,15 @@ def cmake_args(self):
         if self.spec.satisfies("@2020:"):
             options.append("-DGMX_INSTALL_LEGACY_API=ON")
 
-        if self.spec.satisfies("%oneapi"):
+        if self.spec.satisfies("%oneapi") or self.spec.satisfies("%intel"):
+            # If intel-oneapi-compilers was installed through spack the gcc is added to the
+            # configuration file.
+            if self.spec.satisfies("+intel_provided_gcc") and os.path.exists(
+                ".".join([os.environ["SPACK_CXX"], "cfg"])
+            ):
+                with open(".".join([os.environ["SPACK_CXX"], "cfg"]), "r") as f:
+                    options.append("-DCMAKE_CXX_FLAGS={}".format(f.read()))
+            else:
                 options.append("-DGMX_GPLUSPLUS_PATH=%s/g++" % self.spec["gcc"].prefix.bin)
 
         if "+double" in self.spec: