Try find_package(MPI REQUIRED) for MPI build

The compiled MPI executable breaks binary equivalence.

The compiled serial executable preserves binary equivalence.

Shared object files between MPI and serial builds are identical, CMake
object libraries could be useful here.

CMakeLists.txt

@@ -11,6 +11,9 @@ option(CABLE_MPI "Build the MPI executable" OFF)
 # third party libs
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(NETCDF REQUIRED IMPORTED_TARGET "netcdf-fortran")
+if(CABLE_MPI)
+    find_package(MPI REQUIRED)
+endif()
 
 set(CABLE_INTEL_Fortran_FLAGS -fp-model precise)
 set(CABLE_INTEL_Fortran_FLAGS_DEBUG -O0 -g -traceback -fpe0)
@@ -166,6 +169,7 @@ endif()
 target_link_libraries(
     cable
     PkgConfig::NETCDF
+    "$<$<BOOL:${CABLE_MPI}>:MPI::MPI_Fortran>"
 )
 
 install(TARGETS cable RUNTIME)

build.bash

@@ -68,7 +68,7 @@ else
 fi
 if [[ -n $mpi ]]; then
     cmake_args+=(-DCABLE_MPI="ON")
-    cmake_args+=(-DCMAKE_Fortran_COMPILER="mpif90")
+    cmake_args+=(-DMPI_Fortran_COMPILER="mpif90")
 fi
 
 cmake_build_args=()