updated setup

setup.py

@@ -96,7 +96,7 @@ binary_c_python_api_module = Extension(
         "include", 
     ]
     + binary_c_incdirs
-    + [GSL_DIR] if GSL_DIR else [],
+    + [os.path.join(GSL_DIR,'include')] if GSL_DIR else [],
     libraries=["binary_c"] + binary_c_libs + ["binary_c_api"],
     library_dirs=[
         os.environ["BINARY_C"] + "/src",