Warn when doing an out-of-source build and finding in-source build artifacts

The reason to warn is that configuration *may* pick up on
configuration header files that are in the source tree, that might be
for a wildly different configuration than what is expected in the
current out-of-source configuration.
Reviewed-by: NMatt Caswell <matt@openssl.org>

Configure

@@ -1923,6 +1923,14 @@ or position independent code, please let us know (but please first make sure
 you have tried with a current version of OpenSSL).
 EOF
 
+print <<"EOF" if (-f catfile($srcdir, "configdata.pm") && $srcdir ne $blddir);
+
+WARNING: there are indications that another build was made in the source
+directory.  This build may have picked up artifacts from that build, the
+safest course of action is to clean the source directory and redo this
+configuration.
+EOF
+
 exit(0);
 
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