[BUILD] when using -DFREECAD_USE_EXTERNAL_ONDSELSOLVER there should be a check of external solver interface

https://github.com/FreeCAD/FreeCAD/blob/c9041132f97c7e0c432e16d68970d68738de43ec/src/Mod/Assembly/App/AssemblyObject.cpp#L238

I am trying to build freecad but I get the following error:

```
FAILED: [code=1] src/Mod/Assembly/App/CMakeFiles/Assembly.dir/AssemblyObject.cpp.o
/usr/bin/x86_64-pc-linux-gnu-g++ -DAssembly_EXPORTS -DBOOST_DATE_TIME_DYN_LINK -DBOOST_DATE_TIME_NO_LIB -DBOOST_PROGRAM_OPTIONS_DYN_LINK -DBOOST_PROGRAM_OPTIONS_NO_LIB -DBOOST_REGEX_DYN_LINK -DBOOST_REGEX_NO_LIB -DBOOST_THREAD_DYN_LINK -DBOOST_THREAD_NO_LIB -DFMT_SHARED -DHAVE_CONFIG_H -DHAVE_OPENGL_EXT -DHAVE_QT -DHAVE_RAPIDJSON -DHAVE_TBB -DHAVE_TK -DHAVE_VTK -DHAVE_XLIB -DOCC_CONVERT_SIGNALS -DPYCXX_6_2_COMPATIBILITY -DQT_CONCURRENT_LIB -DQT_CORE_LIB -DQT_NO_DEBUG -DQT_NO_KEYWORDS -DQT_XML_LIB -DVTK_OPENGL2_BACKEND -D_OCC64 -I/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999_build/src/Mod/Assembly/App/Assembly_autogen/include -I/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999_build -I/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999_build/src -I/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999/src -I/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999_build/src/Mod/Assembly/App -I/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999/src/3rdParty/FastSignals/libfastsignals/include -I/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999/src/3rdParty/lru-cache/include -isystem /usr/include/opencascade -isystem /usr/include/freetype2 -isystem /usr/include/qt6/QtCore -isystem /usr/include/qt6 -isystem /usr/include/qt6/QtXml -isystem /usr/include/python3.12 -isystem /var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999/src/3rdParty -isystem /usr/lib64/qt6/mkspecs/linux-g++ -isystem /usr/include/qt6/QtConcurrent -Wall -Wextra -Wno-write-strings -fdiagnostics-color -O2 -pipe -march=znver3 -ggdb -fno-strict-aliasing -std=c++20 -fPIC -mno-direct-extern-access -MD -MT src/Mod/Assembly/App/CMakeFiles/Assembly.dir/AssemblyObject.cpp.o -MF src/Mod/Assembly/App/CMakeFiles/Assembly.dir/AssemblyObject.cpp.o.d -o src/Mod/Assembly/App/CMakeFiles/Assembly.dir/AssemblyObject.cpp.o -c /var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999/src/Mod/Assembly/App/AssemblyObject.cpp
/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999/src/Mod/Assembly/App/AssemblyObject.cpp: In lambda function:
/var/tmp/portage/media-gfx/freecad-9999/work/freecad-9999/src/Mod/Assembly/App/AssemblyObject.cpp:238:37: error: ‘using std::__shared_ptr_access::element_type = class MbD::Constraint’ {aka ‘class MbD::Constraint’} has no member named ‘constraintSpec’; did you mean ‘Constraint’?
238 | std::string spec = con->constraintSpec();
| ^~~~~~~~~~~~~~
| Constraint

```

the code was introduced two days ago https://github.com/FreeCAD/FreeCAD/pull/24623 @PaddleStroke @kadet1090