pythongh-125269: Use AC_LINK_IF_ELSE to detect if -latomic is nee…

…ded (python#125416)

We previously used `AC_RUN_IF_ELSE` with a short test program to detect
if `-latomic` is needed, but that requires choosing a specific default
value when cross-compiling because the test program is not run.
Some cross compilation targets like `wasm32-emscripten` do not support
`-latomic`, while other cross compilation targets, like
`arm-linux-gnueabi` require it.

Misc/NEWS.d/next/Build/2024-10-13-21-11-30.gh-issue-125269.BC-fdo.rst

@@ -0,0 +1,2 @@
+Fix detection of whether ``-latomic`` is needed when cross-compiling CPython
+using the configure script.

configure.ac

@@ -7497,7 +7497,7 @@ CPPFLAGS="${BASECPPFLAGS} -I. -I${srcdir}/Include ${CPPFLAGS}"
 
 AC_CACHE_CHECK([whether libatomic is needed by <pyatomic.h>],
                [ac_cv_libatomic_needed],
-[AC_RUN_IFELSE([AC_LANG_SOURCE([[
+[AC_LINK_IFELSE([AC_LANG_SOURCE([[
 // pyatomic.h needs uint64_t and Py_ssize_t types
 #include <stdint.h>  // int64_t, intptr_t
 #ifdef HAVE_SYS_TYPES_H
@@ -7534,9 +7534,8 @@ int main()
     return 0; // all good
 }
 ]])],
-  [ac_cv_libatomic_needed=no],  dnl build succeeded
-  [ac_cv_libatomic_needed=yes], dnl build failed
-  [ac_cv_libatomic_needed=no])  dnl cross compilation
+  [ac_cv_libatomic_needed=no],  dnl build and link succeeded
+  [ac_cv_libatomic_needed=yes]) dnl build and link failed
 ])
 
 AS_VAR_IF([ac_cv_libatomic_needed], [yes],