pythongh-124932: Cross builds: Distinguish between build prefix and h…

…ost prefix

In Emscripten and wasi builds, and presumably for 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. `wasi.py` has to mess around with setting `PYTHONPATH`
because `prefix` is set to a path from the build machine. It would simplify
this if we distinguish 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`.

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@
 
@@ -1737,8 +1744,8 @@ 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)"' \

configure.ac

@@ -379,6 +379,27 @@ 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=/],
+    [WASI], [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],
+    [WASI], [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.
 #