added C example

cextern/helloworld.c

+// Simple C program to display "Hello World"
+
+// Header file for input output functions
+#include <stdio.h>
+
+// main function -
+// where the execution of program begins
+int main()
+{
+
+    // prints hello world
+    printf("Hello World");
+
+    return 0;
+}
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install_local.sh

 #!/usr/bin/env bash
 rm -rf build
 rm -rf dist
-#python setup.py build_ext --inplace
+python setup.py build_ext
 #python setup.py install
 python setup.py sdist
 pip install dist/*.tar.gz --force-reinstall

setup.py

@@ -37,4 +37,9 @@ setuptools.setup(
     #                   'astropy',
     #                   ],
     python_requires='>=3.9',
+    ext_modules=[
+        setuptools.Extension(
+            name="csst_proto.cext.helloworld", sources=["cextern/helloworld.c"]
+        )
+    ]
 )
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