cmake: set sundials path hint for python package (#2397)

So far, CMakeLists.txt only provided a path hint for sundials for the
standalone c++ library, but not for the python package. The latter
should be added to ensure the right sundials version is found. There
have been issues with system installations of sundials.

Closes #2394

CMakeLists.txt

@@ -133,9 +133,16 @@ elseif(AMICI_TRY_ENABLE_HDF5)
 endif()
 
 set(VENDORED_SUNDIALS_DIR ${CMAKE_CURRENT_SOURCE_DIR}/ThirdParty/sundials)
-set(VENDORED_SUNDIALS_BUILD_DIR ${VENDORED_SUNDIALS_DIR}/build)
-set(VENDORED_SUNDIALS_INSTALL_DIR ${VENDORED_SUNDIALS_BUILD_DIR})
 set(SUNDIALS_PRIVATE_INCLUDE_DIRS "${VENDORED_SUNDIALS_DIR}/src")
+# Handle different sundials build/install dirs, depending on whether we are
+#  building the Python extension only or the full C++ interface
+if(AMICI_PYTHON_BUILD_EXT_ONLY)
+  set(VENDORED_SUNDIALS_BUILD_DIR ${CMAKE_CURRENT_SOURCE_DIR})
+  set(VENDORED_SUNDIALS_INSTALL_DIR ${VENDORED_SUNDIALS_BUILD_DIR})
+else()
+  set(VENDORED_SUNDIALS_BUILD_DIR ${VENDORED_SUNDIALS_DIR}/build)
+  set(VENDORED_SUNDIALS_INSTALL_DIR ${VENDORED_SUNDIALS_BUILD_DIR})
+endif()
 find_package(
   SUNDIALS REQUIRED PATHS
   "${VENDORED_SUNDIALS_INSTALL_DIR}/${CMAKE_INSTALL_LIBDIR}/cmake/sundials/")