gh-124932: Distinguish build prefix from host prefix in cross builds (#124933)

In Emscripten and other cross builds, the build file system and the host file
system look different. For instance, we may want to install into
`cross-build/$TARGET/lib`, and then mount that as `/lib` in the host file
system. This change adds a distinction between:

* `prefix` -- the path in the build file system where we want to install the files
* `host_prefix` -- the path in the host file system where getpath.c will look for the files

And similarly for `exec_prefix` and `host_exec_prefix`. At present, this is only
used for Emscripten.

Author: hoodmane

Makefile.pre.in

@@ -141,6 +141,13 @@ prefix=		@prefix@
 # Install prefix for architecture-dependent files
 exec_prefix=	@exec_prefix@
 
+# For cross compilation, we distinguish between "prefix" (where we install the
+# files) and "host_prefix" (where getpath.c expects to find the files at
+# runtime)
+host_prefix= 	@host_prefix@
+host_exec_prefix= 	@host_exec_prefix@
+
+
 # Install prefix for data files
 datarootdir=    @datarootdir@
 
@@ -1740,8 +1747,8 @@ Modules/getbuildinfo.o: $(PARSER_OBJS) \
 
 Modules/getpath.o: $(srcdir)/Modules/getpath.c Python/frozen_modules/getpath.h Makefile $(PYTHON_HEADERS)
 	$(CC) -c $(PY_CORE_CFLAGS) -DPYTHONPATH='"$(PYTHONPATH)"' \
-		-DPREFIX='"$(prefix)"' \
-		-DEXEC_PREFIX='"$(exec_prefix)"' \
+		-DPREFIX='"$(host_prefix)"' \
+		-DEXEC_PREFIX='"$(host_exec_prefix)"' \
 		-DVERSION='"$(VERSION)"' \
 		-DVPATH='"$(VPATH)"' \
 		-DPLATLIBDIR='"$(PLATLIBDIR)"' \

Misc/NEWS.d/next/Build/2024-10-25-17-20-50.gh-issue-124932.F-aNuS.rst

@@ -0,0 +1,4 @@
+For cross builds, there is now support for having a different install
+``prefix`` than the ``host_prefix`` used by ``getpath.py``. This is set to ``/`` by
+default for Emscripten, on other platforms the default behavior is the same
+as before.

configure

@@ -379,6 +379,25 @@ then
 fi
 AC_MSG_RESULT(["$MACHDEP"])
 
+dnl For cross compilation, we distinguish between "prefix" (where we install the
+dnl files) and "host_prefix" (where we expect to find the files at runtime)
+
+if test -z "$host_prefix"; then
+  AS_CASE([$ac_sys_system],
+    [Emscripten], [host_prefix=/],
+    [host_prefix='${prefix}']
+  )
+fi
+AC_SUBST([host_prefix])
+
+if test -z "$host_exec_prefix"; then
+  AS_CASE([$ac_sys_system],
+    [Emscripten], [host_exec_prefix=$host_prefix],
+    [host_exec_prefix='${exec_prefix}']
+  )
+fi
+AC_SUBST([host_exec_prefix])
+
 # On cross-compile builds, configure will look for a host-specific compiler by
 # prepending the user-provided host triple to the required binary name.
 #