GH-145273: drop build_prefix check from missing stdlib warning (#145437)

Author: FFY00

Makefile.pre.in

@@ -1004,7 +1004,7 @@ platform: $(PYTHON_FOR_BUILD_DEPS) pybuilddir.txt
 # or removed in case of failure.
 pybuilddir.txt: $(PYTHON_FOR_BUILD_DEPS)
 	@echo "none" > ./pybuilddir.txt
-	$(RUNSHARED) $(PYTHON_FOR_BUILD) -S -m sysconfig --generate-posix-vars ;\
+	$(RUNSHARED) $(PYTHON_FOR_BUILD) -S -X pathconfig_warnings=0 -m sysconfig --generate-posix-vars ;\
 	if test $$? -ne 0 ; then \
 		echo "generate-posix-vars failed" ; \
 		rm -f ./pybuilddir.txt ; \

Modules/getpath.py

@@ -779,7 +779,7 @@ def search_up(prefix, *landmarks, test=isfile):
     if not stdlib_dir or not isdir(stdlib_dir):
         hint = home_hint if home else f'sys.prefix is set to {prefix}, is this correct?'
         warn('WARN: Could not find the standard library directory! ' + hint)
-    elif (not platstdlib_dir and not build_prefix) or not isdir(platstdlib_dir):
+    elif not platstdlib_dir or not isdir(platstdlib_dir):
         hint = home_hint if home else f'sys.exec_prefix is set to {exec_prefix}, is this correct?'
         warn('WARN: Could not find the platform standard library directory! ' + hint)