diff --git a/Makefile.pre.in b/Makefile.pre.in

--- a/Makefile.pre.in

+++ b/Makefile.pre.in

@@ -71,12 +71,17 @@

 BASECFLAGS=	@BASECFLAGS@

 BASECPPFLAGS=	@BASECPPFLAGS@

 CONFIGURE_CFLAGS=	@CFLAGS@

+# CFLAGS_NODIST is used for building the interpreter and stdlib C extensions.

+# Use it when a compiler flag should _not_ be part of the distutils CFLAGS

+# once Python is installed (Issue #21121).

+CONFIGURE_CFLAGS_NODIST=@CFLAGS_NODIST@

 CONFIGURE_CPPFLAGS=	@CPPFLAGS@

 CONFIGURE_LDFLAGS=	@LDFLAGS@

 # Avoid assigning CFLAGS, LDFLAGS, etc. so users can use them on the

 # command line to append to these values without stomping the pre-set

 # values.

 PY_CFLAGS=	$(BASECFLAGS) $(OPT) $(CONFIGURE_CFLAGS) $(CFLAGS) $(EXTRA_CFLAGS)

+PY_CFLAGS_NODIST=$(CONFIGURE_CFLAGS_NODIST) $(CFLAGS_NODIST)

 # Both CPPFLAGS and LDFLAGS need to contain the shell's value for setup.py to

 # be able to build extension modules using the directories specified in the

 # environment variables

@@ -91,7 +96,7 @@

 # Extra C flags added for building the interpreter object files.

 CFLAGSFORSHARED=@CFLAGSFORSHARED@

 # C flags used for building the interpreter object files

-PY_CORE_CFLAGS=	$(PY_CFLAGS) $(PY_CPPFLAGS) $(CFLAGSFORSHARED) -DPy_BUILD_CORE

+PY_CORE_CFLAGS=	$(PY_CFLAGS) $(PY_CFLAGS_NODIST) $(PY_CPPFLAGS) $(CFLAGSFORSHARED) -DPy_BUILD_CORE

 # Machine-dependent subdirectories

diff --git a/configure b/configure

--- a/configure

+++ b/configure

@@ -662,6 +662,7 @@

 LIBTOOL_CRUFT

 OTHER_LIBTOOL_OPT

 UNIVERSAL_ARCH_FLAGS

+CFLAGS_NODIST

 BASECFLAGS

 OPT

 ABIFLAGS

@@ -6504,7 +6505,7 @@

     if test $ac_cv_declaration_after_statement_warning = yes

     then

-      BASECFLAGS="$BASECFLAGS -Werror=declaration-after-statement"

+      CFLAGS_NODIST="$CFLAGS_NODIST -Werror=declaration-after-statement"

     fi

     # if using gcc on alpha, use -mieee to get (near) full IEEE 754

diff --git a/configure.ac b/configure.ac

--- a/configure.ac

+++ b/configure.ac

@@ -1147,6 +1147,7 @@

 fi

 AC_SUBST(BASECFLAGS)

+AC_SUBST(CFLAGS_NODIST)

 # The -arch flags for universal builds on OSX

 UNIVERSAL_ARCH_FLAGS=

@@ -1231,7 +1232,7 @@

     if test $ac_cv_declaration_after_statement_warning = yes

     then

-      BASECFLAGS="$BASECFLAGS -Werror=declaration-after-statement"

+      CFLAGS_NODIST="$CFLAGS_NODIST -Werror=declaration-after-statement"

     fi

     # if using gcc on alpha, use -mieee to get (near) full IEEE 754

diff --git a/setup.py b/setup.py

--- a/setup.py

+++ b/setup.py

@@ -19,6 +19,12 @@

 cross_compiling = "_PYTHON_HOST_PLATFORM" in os.environ

+# Add special CFLAGS reserved for building the interpreter and the stdlib

+# modules (Issue #21121).

+cflags = sysconfig.get_config_var('CFLAGS')

+py_cflags_nodist = sysconfig.get_config_var('PY_CFLAGS_NODIST')

+sysconfig.get_config_vars()['CFLAGS'] = cflags + ' ' + py_cflags_nodist

+

 def get_platform():

     # cross build

     if "_PYTHON_HOST_PLATFORM" in os.environ: