Fix check_emscripten_port() function.

Match the autoconf macro behavior: set per-package variables
(ZLIB_CFLAGS, ZLIB_LIBS, etc.) only when the system is Emscripten and
the user hasn't already provided values.

Author: nascheme

Tools/configure/pyconf.py

@@ -3233,19 +3233,25 @@ def pkg_check_modules(pkg_var: str, module_spec: str) -> Any:
 
 
 def check_emscripten_port(pkg_var: str, emport_args: str) -> None:
-    """Check for an Emscripten port (e.g. USE_ZLIB). Sets CFLAGS/LDFLAGS.
+    """Check for an Emscripten port (e.g. USE_ZLIB).
 
-    Only active when CC is emcc; otherwise a no-op.
+    Sets ``{pkg_var}_CFLAGS`` and ``{pkg_var}_LIBS`` to *emport_args*
+    when building with emcc and the user hasn't already provided values.
+    Matches autoconf's PY_CHECK_EMSCRIPTEN_PORT macro.
+    Only active when ac_sys_system is Emscripten; otherwise a no-op.
     """
-    if "emcc" not in CC:
+    if vars.ac_sys_system != "Emscripten":
         return
     flag = (
         f"-s{emport_args}" if not emport_args.startswith("-") else emport_args
     )
-    existing_cflags = substs.get("CFLAGS", "")
-    existing_ldflags = substs.get("LDFLAGS", "")
-    substs["CFLAGS"] = f"{existing_cflags} {flag}".strip()
-    substs["LDFLAGS"] = f"{existing_ldflags} {flag}".strip()
+    cflags_name = f"{pkg_var}_CFLAGS"
+    libs_name = f"{pkg_var}_LIBS"
+    # Only set if user hasn't already provided values (Vars.__getattr__
+    # returns "" for unset attributes, matching shell empty-variable checks).
+    if not getattr(vars, cflags_name) and not getattr(vars, libs_name):
+        setattr(vars, cflags_name, flag)
+        setattr(vars, libs_name, flag)
 
 
 # ---------------------------------------------------------------------------

Tools/configure/transpiler/pyconf.awk

@@ -2006,9 +2006,18 @@ function pyconf_find_compiler(user_cc, user_cpp, cc, cpp, ver) {
         pyconf_ac_cv_prog_cc_g = "yes"
 }
 
-function pyconf_check_emscripten_port(port, name) {
-        # Stub for emscripten port checks
-        return 0
+function pyconf_check_emscripten_port(pkg_var, emport_args,    flag, cname, lname) {
+        # Set {pkg_var}_CFLAGS and {pkg_var}_LIBS to emport_args when
+        # building with Emscripten and user hasn't provided values.
+        if (V["ac_sys_system"] != "Emscripten") return
+        flag = emport_args
+        if (substr(flag, 1, 1) != "-") flag = "-s" flag
+        cname = pkg_var "_CFLAGS"
+        lname = pkg_var "_LIBS"
+        if (V[cname] == "" && V[lname] == "") {
+                V[cname] = flag
+                V[lname] = flag
+        }
 }
 
 function pyconf_ax_c_float_words_bigendian(on_big, on_little, on_unknown, src, conftest, exe, cmd, rc, has_big, has_little, pre, post) {