Improve SCOTCH handling in CMake (#3705)

* Refine CMake/SCOTCH handling

* Tidy

Author: garth-wells

cpp/CMakeLists.txt

@@ -237,7 +237,6 @@ set_package_properties(
 # Check for MPI
 find_package(MPI 3 REQUIRED)
 
-find_package(spdlog REQUIRED)
 # ------------------------------------------------------------------------------
 # Compiler flags
 
@@ -290,8 +289,8 @@ endif()
 # ------------------------------------------------------------------------------
 # Find required packages
 
-# pugixml
 find_package(pugixml REQUIRED)
+find_package(spdlog REQUIRED)
 
 # Note: When updating Boost version, also update DOLFINXConfig.cmake.in
 if(DEFINED ENV{BOOST_ROOT} OR DEFINED BOOST_ROOT)
@@ -464,29 +463,31 @@ elseif(_REQUIRE_SLEPC AND NOT PETSC_FOUND)
   )
 endif()
 
-if(DOLFINX_ENABLE_SCOTCH)
+if(DOLFINX_ENABLE_SCOTCH AND _REQUIRE_SCOTCH)
+  find_package(
+    SCOTCH REQUIRED CONFIG HINTS $ENV{PETSC_DIR}/$ENV{PETSC_ARCH}
+    $ENV{PETSC_DIR}
+  )
+elseif(DOLFINX_ENABLE_SCOTCH)
   find_package(
     SCOTCH CONFIG HINTS $ENV{PETSC_DIR}/$ENV{PETSC_ARCH} $ENV{PETSC_DIR}
   )
-  if(TARGET SCOTCH::scotch AND NOT TARGET SCOTCH::ptscotch)
-    message(
-      STATUS "SCOTCH found, but not PT-SCOTCH (parallel). Not enabling SCOTCH."
-    )
-    set(SCOTCH_FOUND FALSE)
-  endif()
+endif()
 
-  if(SCOTCH_FOUND)
-    message(
-      STATUS
-        "Found PT-SCOTCH version ${SCOTCH_VERSION}, include dir: ${SCOTCH_INCLUDE_DIR}, lib dir: ${SCOTCH_LIBRARY_DIR}"
-    )
+if(TARGET SCOTCH::scotch AND NOT TARGET SCOTCH::ptscotch)
+  message(
+    STATUS "SCOTCH found, but not PT-SCOTCH (parallel). Not enabling SCOTCH."
+  )
+  set(SCOTCH_FOUND FALSE)
+  if(_REQUIRE_SCOTCH AND NOT SCOTCH_FOUND)
+    message(FATAL_ERROR "SCOTCH requested, but not found.")
   endif()
 endif()
 
-if(_REQUIRE_SCOTCH AND NOT SCOTCH_FOUND)
+if(SCOTCH_FOUND)
   message(
-    FATAL_ERROR
-      "SCOTCH requested, but not found."
+    STATUS
+      "Found PT-SCOTCH version ${SCOTCH_VERSION}, include dir: ${SCOTCH_INCLUDE_DIR}, lib dir: ${SCOTCH_LIBRARY_DIR}"
   )
 endif()