gh-117889: Fix PGO test in test_peg_generator

Reuse support.check_cflags_pgo() in test_peg_generator to check if
PGO build is used or not.

Author: vstinner

Lib/test/test_peg_generator/test_c_parser.py

@@ -13,9 +13,7 @@
 from test.support import os_helper, import_helper
 from test.support.script_helper import assert_python_ok
 
-_py_cflags_nodist = sysconfig.get_config_var("PY_CFLAGS_NODIST")
-_pgo_flag = sysconfig.get_config_var("PGO_PROF_USE_FLAG")
-if _pgo_flag and _py_cflags_nodist and _pgo_flag in _py_cflags_nodist:
+if support.check_cflags_pgo():
     raise unittest.SkipTest("peg_generator test disabled under PGO build")
 
 test_tools.skip_if_missing("peg_generator")