bpo-41916: allow cross-compiled python to have -pthread set for CXX

When cross-compiling, the compile/run test for -pthread always fails so -pthread
will never be automatically set without an override from the cache. ac_cv_pthread
can already be overridden, so do the same thing for ac_cv_cxx_thread.

Author: virtuald

Misc/NEWS.d/next/Build/2022-03-04-10-47-23.bpo-41916.1d2GLU.rst

@@ -0,0 +1,2 @@
+Allow override of ac_cv_cxx_thread so that cross compiled python can set
+-pthread for CXX.

configure

@@ -2664,11 +2664,10 @@ fi
 
 # If we have set a CC compiler flag for thread support then
 # check if it works for CXX, too.
-ac_cv_cxx_thread=no
 if test ! -z "$CXX"
 then
-AC_MSG_CHECKING(whether $CXX also accepts flags for thread support)
-ac_save_cxx="$CXX"
+AC_CACHE_CHECK([whether $CXX also accepts flags for thread support], [ac_cv_cxx_thread],
+[ac_save_cxx="$CXX"
 
 if test "$ac_cv_kpthread" = "yes"
 then
@@ -2697,9 +2696,10 @@ then
   fi
   rm -fr conftest*
 fi
-AC_MSG_RESULT($ac_cv_cxx_thread)
+CXX="$ac_save_cxx"])
+else
+  ac_cv_cxx_thread=no
 fi
-CXX="$ac_save_cxx"
 
 dnl # check for ANSI or K&R ("traditional") preprocessor
 dnl AC_MSG_CHECKING(for C preprocessor type)