Do not run the code generator if PYERFA_USE_SYSTEM_LIBERFA=1

setup.py

@@ -10,6 +10,10 @@
 
 LIBERFADIR = os.path.join('liberfa', 'erfa')
 ERFA_SRC = os.path.join(LIBERFADIR, 'src')
+GEN_FILES = [
+    os.path.join('erfa', 'core.py'),
+    os.path.join('erfa', 'ufunc.c'),
+]
 
 
 # https://mail.python.org/pipermail/distutils-sig/2007-September/008253.html
@@ -27,18 +31,25 @@ def include_dirs(self, include_dirs):
 
 
 def get_extensions():
-    cmd = [sys.executable, 'erfa_generator.py', ERFA_SRC, '--quiet']
-    subprocess.run(cmd, check=True)
-
     sources = [os.path.join('erfa', 'ufunc.c')]
-
     include_dirs = []
-
     libraries = []
 
     if int(os.environ.get('PYERFA_USE_SYSTEM_LIBERFA', 0)):
         libraries.append('erfa')
+
+        if not all(os.path.isfile(fn) for fn in GEN_FILES):
+            raise RuntimeError(
+                'Missing "liberfa" source files, unable to generate '
+                '"erfa/ufunc.c" and "erfa/core.py". '
+                'Please check your source tree. '
+                'Maybe "git submodule update" could help.')
+
     else:
+        print('Run "erfa_generator.py"')
+        cmd = [sys.executable, 'erfa_generator.py', ERFA_SRC, '--quiet']
+        subprocess.run(cmd, check=True)
+
         # get all of the .c files in the liberfa/erfa/src directory
         erfafns = os.listdir(ERFA_SRC)
         sources.extend([os.path.join(ERFA_SRC, fn)