Support packages which need to explicitly refer to dpcpp by name (#27168)

* Hack to support packages which need to explicitly refer to dpcpp by name
* cc script needs to know about dpcpp

lib/spack/env/cc

@@ -248,7 +248,7 @@ case "$command" in
         lang_flags=C
         debug_flags="-g"
         ;;
-    c++|CC|g++|clang++|armclang++|icpc|icpx|pgc++|nvc++|xlc++|xlc++_r|FCC)
+    c++|CC|g++|clang++|armclang++|icpc|icpx|dpcpp|pgc++|nvc++|xlc++|xlc++_r|FCC)
         command="$SPACK_CXX"
         language="C++"
         comp="CXX"

lib/spack/env/oneapi/dpcpp

@@ -0,0 +1 @@
+../cc

lib/spack/spack/compilers/dpcpp.py

@@ -0,0 +1,29 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+import spack.compilers.oneapi
+
+
+class Dpcpp(spack.compilers.oneapi.Oneapi):
+    """This is the same as the oneAPI compiler but uses dpcpp instead of
+    icpx (for DPC++ source files). It explicitly refers to dpcpp, so that
+    CMake test files which check the compiler name (e.g. CMAKE_CXX_COMPILER)
+    detect it as dpcpp.
+
+    Ideally we could switch out icpx for dpcpp where needed in the oneAPI
+    compiler definition, but two things are needed for that: (a) a way to
+    tell the compiler that it should be using dpcpp and (b) a way to
+    customize the link_paths
+
+    See also: https://www.intel.com/content/www/us/en/develop/documentation/oneapi-dpcpp-cpp-compiler-dev-guide-and-reference/top/compiler-setup/using-the-command-line/invoking-the-compiler.html
+    """
+    # Subclasses use possible names of C++ compiler
+    cxx_names = ['dpcpp']
+
+    # Named wrapper links within build_env_path
+    link_paths = {'cc': 'oneapi/icx',
+                  'cxx': 'oneapi/dpcpp',
+                  'f77': 'oneapi/ifx',
+                  'fc': 'oneapi/ifx'}