cmake fixes for external OndselSolver building (#18379)

* Assembly: fix building with external OndselSolver

This commit pairs with f35b075e271e9cb3451f3439f7dc0274841a0838.
It solves two issues:

- remove hardcoded includes of 3rdparty files
- add missing include_directories()

* Assembly: add check for external OndselSolver includes

src/Mod/Assembly/App/AssemblyObject.h

@@ -33,7 +33,7 @@
 #include <App/PropertyLinks.h>
 #include "SimulationGroup.h"
 
-#include <3rdParty/OndselSolver/OndselSolver/enum.h>
+#include <OndselSolver/enum.h>
 
 namespace MbD
 {

src/Mod/Assembly/App/CMakeLists.txt

@@ -7,11 +7,6 @@ include_directories(
     ${OCC_INCLUDE_DIR}
     ${PYTHON_INCLUDE_DIRS}
 )
-if (NOT FREECAD_USE_EXTERNAL_ONDSELSOLVER)
-include_directories(
-    ${CMAKE_SOURCE_DIR}/src/3rdParty/OndselSolver
-)
-endif ()
 link_directories(${OCC_LIBRARY_DIR})
 
 set(Assembly_LIBS

src/Mod/Assembly/CMakeLists.txt

@@ -1,3 +1,14 @@
+if (NOT FREECAD_USE_EXTERNAL_ONDSELSOLVER)
+include_directories(
+    ${CMAKE_SOURCE_DIR}/src/3rdParty/OndselSolver
+)
+else ()
+    check_include_file_cxx(OndselSolver/enum.h HAVE_ONDSELSOLVER_H)
+    if (NOT HAVE_ONDSELSOLVER_H)
+        message(FATAL_ERROR "FREECAD_USE_EXTERNAL_ONDSELSOLVER is set but the required system includes are not available")
+    endif ()
+endif ()
+
 add_subdirectory(App)
 
 if(BUILD_GUI)