updated cmake (#619)

python/CMakeLists.txt

@@ -1,11 +1,42 @@
 if(FINUFFT_USE_CPU)
-    if (WIN32)
-        install(TARGETS finufft LIBRARY DESTINATION finufft RUNTIME DESTINATION finufft)
-    else ()
-        install(TARGETS finufft LIBRARY DESTINATION finufft)
-    endif ()
+  if(WIN32)
+    install(
+      TARGETS finufft
+      LIBRARY DESTINATION finufft
+      RUNTIME DESTINATION finufft)
+  else()
+    install(TARGETS finufft LIBRARY DESTINATION finufft)
+  endif()
 endif()
 
 if(FINUFFT_USE_CUDA)
-    install(TARGETS cufinufft LIBRARY DESTINATION cufinufft)
+  install(TARGETS cufinufft LIBRARY DESTINATION cufinufft)
+endif()
+
+# Warn if the user invokes CMake directly
+if(NOT SKBUILD)
+  message(
+    WARNING
+      "\
+  This CMake file is meant to be executed using 'scikit-build-core'.
+  Running it directly will almost certainly not produce the desired
+  result. If you are a user trying to install this package, use the
+  command below, which will install all necessary build dependencies,
+  compile the package in an isolated environment, and then install it.
+  =====================================================================
+   $ pip install python/finufft
+   or
+   $ pip install python/cufinufft
+  =====================================================================
+  If you are a software developer, and this is your own package, then
+  it is usually much more efficient to install the build dependencies
+  in your environment once and use the following command that avoids
+  a costly creation of a new virtual environment at every compilation:
+  =====================================================================
+   $ pip install finufft scikit-build-core[pyproject]
+   $ pip install --no-build-isolation -ve .
+  =====================================================================
+  You may optionally add -Ceditable.rebuild=true to auto-rebuild when
+  the package is imported. Otherwise, you need to rerun the above
+  after editing C++ files.")
 endif()