Define get_lines outside of the if statement

pytensor/link/c/cmodule.py

@@ -2104,9 +2104,6 @@ class GCC_compiler(Compiler):
             )
             detect_march = False
 
-        if detect_march:
-            GCC_compiler.march_flags = []
-
         def get_lines(cmd, parse=True):
             p = subprocess_Popen(
                 cmd,
@@ -2144,6 +2141,9 @@ class GCC_compiler(Compiler):
 
             return lines
 
+        if detect_march:
+            GCC_compiler.march_flags = []
+
             # The '-' at the end is needed. Otherwise, g++ do not output
             # enough information.
             native_lines = get_lines(f"{config.cxx} -march=native -E -v -")