python3.13 fails to build on macOS 26

[Greg Troxel <gdt%lexort.com@localhost>]

2020ish M1 mac, latest OS.  pkgsrc head.

pkgsrc expat is not installed.  bl3 finds system expat

  sys   BUILDLINK_PKGNAME.expat                 # undefined
  sys   BUILDLINK_PREFIX.expat=                 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr
  sys   BUILDLINK_API_DEPENDS.expat=            expat>=1.95.7
  sys   BUILDLINK_ABI_DEPENDS.expat=            expat>=2.0.0nb1
  sys   BUILDLINK_AUTO_VARS.expat               # undefined
  sys   BUILDLINK_BUILTIN_MK.expat=             ../../textproc/expat/builtin.mk
  sys   BUILDLINK_CONTENTS_FILTER.expat         # undefined
  sys   BUILDLINK_CPPFLAGS.expat                # undefined
  sys   BUILDLINK_DEPMETHOD.expat=              full
  sys   BUILDLINK_FILES_CMD.expat               # undefined
  sys   BUILDLINK_INCDIRS.expat                 # undefined
  sys   BUILDLINK_LDFLAGS.expat                 # undefined
  sys   BUILDLINK_LIBDIRS.expat                 # undefined
  sys   BUILDLINK_RPATHDIRS.expat               # undefined
  sys   IGNORE_PKG.expat                        # undefined
  sys   USE_BUILTIN.expat=                      yes
                                                expat \
                                --with-system-expat \

and then expat_config.h is not in bl3 and the build doesn't find it.

84 other packages built ok.

I suspect we need to allow the system include path to be bl3ed but I'm
fuzzy beyond that thought.

Does this build for others?


===> Building for python313-3.13.14
clang  -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -O2 -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/opt/pkg/include -DNCURSES_WIDECHAR=1 -O2 -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/opt/pkg/include -DNCURSES_WIDECHAR=1 -O2 -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/opt/pkg/include -DNCURSES_WIDECHAR=1  -std=gnu11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include -I/opt/pkg/include -DNCURSES_WIDECHAR=1 -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/opt/pkg/include -DNCURSES_WIDECHAR=1 -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include   -c ./Modules/pyexpat.c -o Modules/pyexpat.o
./Modules/pyexpat.c:14:10: fatal error: 'expat_config.h' file not found
#include "expat_config.h"
         ^~~~~~~~~~~~~~~~
1 error generated.
gmake: *** [Makefile:3380: Modules/pyexpat.o] Error 1
*** Error code 2