gh-104612: Fix libpython3.so stable ABI on Linux using patchelf

The previous implementation of libpython3.so created an empty wrapper
library that recorded libpython3.X.so in DT_NEEDED but did not properly
re-export symbols. The program with proper Py_LIMITED_API defined cannot
actually linked with libpython3.so (-lpython3), and it makes the stable
API pointless.

And libpython3.so cannot simply be a symbolic link to the
libpython3.X.so, since due to the SONAME contains the minor version,
error will still be reported during the runtime with a different pythons
so version.

This fix uses patchelf on Linux to copy the versioned library and set
its SONAME to libpython3.so, ensuring binaries get the correct
DT_NEEDED entry for stable ABI compatibility.

- Add PATCHELF variable detection in configure.ac (Linux only)
- Require patchelf on Linux, error if not found
- Update libpython3.so target to use patchelf when available
- Fall back to original behavior on non-Linux platforms

A program linked with the newly patched libpython3.so and stable API,
can run with the previous empty wrapper libpython3.so.

Author: beeender

Makefile.pre.in

@@ -67,6 +67,9 @@ SHELL=		/bin/sh -e
 # Use this to make a link between python$(VERSION) and python in $(BINDIR)
 LN=		@LN@
 
+# patchelf is used on Linux to set SONAME of libpython3.so for stable ABI
+PATCHELF=	@PATCHELF@
+
 # Portable install script (configure doesn't always guess right)
 INSTALL=	@INSTALL@
 INSTALL_PROGRAM=@INSTALL_PROGRAM@
@@ -1026,7 +1029,11 @@ libpython$(LDVERSION).so: $(LIBRARY_OBJS) $(DTRACE_OBJS)
 	fi
 
 libpython3.so:	libpython$(LDVERSION).so
-	$(BLDSHARED) $(NO_AS_NEEDED) -o $@ -Wl,-h$@ $^
+	if test -n "$(PATCHELF)"; then \
+		cp $^ $@ && $(PATCHELF) --set-soname $@ $@; \
+	else \
+		$(BLDSHARED) $(NO_AS_NEEDED) -o $@ -Wl,-h$@ $^; \
+	fi
 
 libpython$(LDVERSION).dylib: $(LIBRARY_OBJS)
 	 $(CC) -dynamiclib $(PY_CORE_LDFLAGS) -undefined dynamic_lookup -Wl,-install_name,$(prefix)/lib/libpython$(LDVERSION).dylib -Wl,-compatibility_version,$(VERSION) -Wl,-current_version,$(VERSION) -o $@ $(LIBRARY_OBJS) $(DTRACE_OBJS) $(SHLIBS) $(LIBC) $(LIBM); \

configure

@@ -1719,6 +1719,17 @@ if test -z "$LN" ; then
 	esac
 fi
 
+# patchelf is required on Linux to set the SONAME of libpython3.so for stable ABI support
+AC_SUBST([PATCHELF])
+case $ac_sys_system in
+	Linux*)
+		AC_CHECK_PROG([PATCHELF], [patchelf], [patchelf], [])
+		if test -z "$PATCHELF"; then
+			AC_MSG_ERROR([patchelf is required on Linux for stable ABI support. Please install patchelf.])
+		fi
+		;;
+esac
+
 # For calculating the .so ABI tag.
 AC_SUBST([ABIFLAGS])
 AC_SUBST([ABI_THREAD])