Handle variable-held complete programs in link_check transpiler.

Author: nascheme

Tools/configure/configure.awk

@@ -8325,7 +8325,7 @@ function u_setup_pthreads(    ac_cv_cxx_thread, ac_cv_kpthread, ac_cv_kthread, a
 		pyconf_result(unistd_defines_pthreads)
 		prog = "#include <stdio.h>\n#include <stdlib.h>\n#include <pthread.h>\nvoid *start_routine(void *arg) { exit(0); }\nint main(void) { pthread_create(NULL, NULL, start_routine, NULL); return 0; }\n"
 		pyconf_checking("for pthread_create in -lpthread")
-		if (pyconf_link_check("", prog "\nint main(void) {  return 0; }", "-lpthread")) {
+		if (pyconf_link_check("", (index(prog, "int main") ? prog : prog "\nint main(void) {  return 0; }"), "-lpthread")) {
 			pyconf_result("yes")
 			V["LIBS"] = V["LIBS"] " -lpthread"
 			V["posix_threads"] = "yes"

Tools/configure/transpiler/pysh_to_awk.py

@@ -2760,10 +2760,21 @@ def _build_link_check_call(self, method: str, call: Call) -> A.FuncCall:
         elif len(positional) == 1:
             if _is_complete_program(positional[0]):
                 source = self._expr(positional[0])
-            else:
+            elif isinstance(positional[0], Const):
                 source = self._build_check_source(
                     inc_lines, self._expr(positional[0]), None
                 )
+            else:
+                # Variable — might be a complete program at runtime.
+                # Emit a runtime check mirroring Python link_check's
+                # prologue_is_source detection.
+                var_expr = self._expr(positional[0])
+                wrapped = self._build_check_source(inc_lines, var_expr, None)
+                source = A.Ternary(
+                    A.FuncCall("index", [var_expr, A.StringLit("int main")]),
+                    var_expr,
+                    wrapped,
+                )
 
         # If includes was a variable, prepend runtime-generated #include lines
         if includes_expr is not None: