Give a warning if SO is set. Fixes #610332.

configure

 #! /bin/sh
-# From configure.in Revision: 1.414 .
+# From configure.in Revision: 1.415 .
 # Guess values for system-dependent variables and create Makefiles.
 # Generated by GNU Autoconf 2.53 for python 2.3.
 #
@@ -9478,6 +9478,17 @@ then
 	CYGWIN*)   SO=.dll;;
 	*)	   SO=.so;;
 	esac
+else
+	# this might also be a termcap variable, see #610332
+        echo
+        echo '====================================================================='
+        echo '+                                                                   +'
+	echo '+ WARNING: You have set SO in your environment.                     +'
+        echo '+ Do you really mean to change the extension for shared libraries?  +'
+        echo '+ Continuing in 10 seconds to let you to ponder.                    +'
+        echo '+                                                                   +'
+        echo '====================================================================='
+	sleep 10
 fi
 echo "$as_me:$LINENO: result: $SO" >&5
 echo "${ECHO_T}$SO" >&6

configure.in

@@ -1140,6 +1140,17 @@ then
 	CYGWIN*)   SO=.dll;;
 	*)	   SO=.so;;
 	esac
+else
+	# this might also be a termcap variable, see #610332
+        echo
+        echo '====================================================================='
+        echo '+                                                                   +'
+	echo '+ WARNING: You have set SO in your environment.                     +'
+        echo '+ Do you really mean to change the extension for shared libraries?  +'
+        echo '+ Continuing in 10 seconds to let you to ponder.                    +'
+        echo '+                                                                   +'
+        echo '====================================================================='
+	sleep 10
 fi
 AC_MSG_RESULT($SO)
 # LDSHARED is the ld *command* used to create shared library