CMake - improve how MPI is found and linked

CLAUDE.md

@@ -7,14 +7,14 @@ Rensselaer's Optimistic Simulation System — a parallel discrete-event simulato
 Typical manual configure:
 
 ```
-cmake -S . -B build -DROSS_BUILD_MODELS=ON -DCMAKE_C_COMPILER=mpicc -DCMAKE_BUILD_TYPE=Debug
+cmake -S . -B build -DROSS_BUILD_MODELS=ON -DCMAKE_BUILD_TYPE=Debug
 cmake --build build -j
 cmake --install build
 ```
 
 Models are **not** built by default — pass `-DROSS_BUILD_MODELS=ON` to include `phold` and any symlinked-in models. `BUILD_TESTING=ON` enables the ctest suite.
 
-MPI is required. The top-level `CMakeLists.txt` does per-arch detection via `CMAKE_SYSTEM_PROCESSOR` and falls back to a `gtod` clock if unrecognized. Set `-DROSS_CLOCK_OVERRIDE=YES` to force the gtod clock.
+MPI is required and auto-discovered via `find_package(MPI)` — do not set `CC=mpicc` or `-DCMAKE_C_COMPILER=mpicc`. For non-standard installs, hint with `-DMPI_HOME=...` or `module load <mpi>` before configuring. The top-level `CMakeLists.txt` does per-arch detection via `CMAKE_SYSTEM_PROCESSOR` and falls back to a `gtod` clock if unrecognized. Set `-DROSS_CLOCK_OVERRIDE=YES` to force the gtod clock.
 
 ## Running a model
 

CMakeLists.txt

@@ -183,15 +183,10 @@ ENDIF(VALID_ARCH AND NOT ${ROSS_CLOCK_OVERRIDE})
 
 
 ## MPI
-INCLUDE(SetupMPI)
-IF(MPI_C_FOUND)
-        INCLUDE_DIRECTORIES(${MPI_C_INCLUDE_PATH})
-        LIST(APPEND ROSS_EXTERNAL_LIBS ${MPI_C_LIBRARIES})
-ELSE(MPI_C_FOUND)
-        MESSAGE("WARNING: Could not find MPI!")
-        MESSAGE("         Either add an MPI compiler to your path (using modules)")
-        MESSAGE("         Or force CMake to build using the correct compiler (`export CC=mpicc`)")
-ENDIF(MPI_C_FOUND)
+# MPI is required; build the system C compiler against MPI::MPI_C rather than
+# forcing CC=mpicc. For non-standard installs, hint with -DMPI_HOME=... or
+# `module load <mpi>` before configuring.
+find_package(MPI REQUIRED COMPONENTS C)
 
 # Code coverage instrumentation (GCC/Clang only). Applied at directory scope
 # so flags propagate to core/ and models/. Collect with lcov/gcovr after ctest.

core/CMakeLists.txt

@@ -121,8 +121,10 @@ OPTION(ROSS_BUILD_SHARED_LIBS "Build shared libraries instead of static" OFF)
 SET(BUILD_SHARED_LIBS ${ROSS_BUILD_SHARED_LIBS})
 ADD_LIBRARY(ROSS ${ross_srcs})
 SET_TARGET_PROPERTIES(ROSS PROPERTIES OUTPUT_NAME ROSS)
-TARGET_LINK_LIBRARIES(ROSS ${ROSS_EXTERNAL_LIBS})
-TARGET_INCLUDE_DIRECTORIES(ROSS INTERFACE ${MPI_C_INCLUDE_PATH})
+# MPI is PUBLIC because core/ross.h does `#include <mpi.h>` — consumers must
+# inherit the MPI include path and link against MPI when they link ROSS.
+target_link_libraries(ROSS PUBLIC MPI::MPI_C)
+target_link_libraries(ROSS PRIVATE m)
 
 # Build Specific Config Header
 CONFIGURE_FILE(config.h.in config.h)