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.

1 files changed, 19 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index a470fd4..42daf1c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -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.
 #