cuda+py: create dummy source for cufinufft C ext

Similarly to what we do for finufft. The issue is that the default gcc
config on certain OSes (Ubuntu 20.04, Rocky 8), libraries are not linked
unless their symbols are explicitly referenced in the source. Since we
have no source, no libraries are linked. The solution is to create a
dummy source that just calls a single function
`cufinufft_default_opts`.

python/cufinufft/setup.py

@@ -4,6 +4,8 @@
 import ctypes
 from pathlib import Path
 
+from tempfile import mkstemp
+
 from setuptools import setup, Extension
 
 # Description
@@ -36,6 +38,28 @@
     raise(e)
 print('cufinufft CUDA shared libraries found, continuing...')
 
+# For certain platforms (e.g. Ubuntu 20.04), we need to create a dummy source
+# that calls one of the functions in the FINUFFT dynamic library. The reason
+# is that these platforms override the default --no-as-needed flag for ld,
+# which means that the linker will only link to those dynamic libraries for
+# which there are unresolved symbols in the object files. Since we do not have
+# a real source, the result is that no dynamic libraries are linked. To
+# prevent this, we create a dummy source so that the library will link as
+# expected.
+fd, source_filename = mkstemp(suffix='.c', text=True)
+
+with open(fd, 'w') as f:
+    f.write( \
+"""
+#include <cufinufft.h>
+
+void PyInit_cufinufftc(void) {
+    cufinufft_opts opt;
+
+    cufinufft_default_opts(&opt);
+}
+""")
+
 
 # Python Package Setup
 setup(
@@ -72,10 +96,13 @@
     py_modules=['cufinufftc'],
     ext_modules=[
         Extension(name='cufinufftc',
-                  sources=[],
+                  sources=[source_filename],
                   libraries=['cufinufft'],
+                  include_dirs=[include_dir],
                   library_dirs=[library_dir],
                   runtime_library_dirs=[library_dir],
                   )
         ]
 )
+
+os.unlink(source_filename)