accept I_MPI_ROOT in place of MPIROOT (#2295)

INSTALL.md

@@ -162,7 +162,7 @@ The build-process (using setup.py) happens in 4 stages:
 ### Configure the Build with Environment Variables
 * ``SKLEARNEX_VERSION``: sets the package version
 * ``DALROOT``: sets the oneAPI Data Analytics Library path
-* ``MPIROOT``: sets the path to the MPI library that will be used for distributed mode support. Not used when using `NO_DIST=1`
+* ``MPIROOT``: sets the path to the MPI library that will be used for distributed mode support. If this variable is not set but `I_MPI_ROOT` is found, will use `I_MPI_ROOT` instead. Not used when using `NO_DIST=1`
 * ``NO_DIST``: set to '1', 'yes' or alike to build without support for distributed mode
 * ``NO_STREAM``: set to '1', 'yes' or alike to build without support for streaming mode
 * ``NO_DPC``: set to '1', 'yes' or alike to build without support of oneDAL DPC++ interfaces

setup.py

@@ -92,7 +92,12 @@
 no_stream = "NO_STREAM" in os.environ and os.environ["NO_STREAM"] in trues
 use_gcov = "SKLEARNEX_GCOV" in os.environ and os.environ["SKLEARNEX_GCOV"] in trues
 debug_build = os.getenv("DEBUG_BUILD") == "1"
-mpi_root = None if no_dist else os.environ["MPIROOT"]
+mpi_root = None if no_dist else os.environ.get("MPIROOT", os.environ.get("I_MPI_ROOT"))
+if (not no_dist) and (mpi_root is None):
+    raise ValueError(
+        "'MPIROOT' is not set, cannot build with distributed mode."
+        " Use 'NO_DIST=1' to build without distributed mode."
+    )
 dpcpp = (
     shutil.which("icpx") is not None
     and "onedal_dpc" in get_onedal_shared_libs(dal_root)