[3.11] gh-94404: Use module CFLAGS before PY_STDMODULE_CFLAGS (GH-94413) (GH-94415)

``PY_STDMODULE_CFLAGS`` may contain include directories with system
headers. This can break compiling with built-in libmpdec.
Co-authored-by: Christian Heimes <christian@python.org>