Configure: count basenames for all library sources

Make sure that each basename only appears once.  This is due to the
static library archiver on Unix, that indexes archived object files by
base name only, thereby making base name clashes...  interesting.

This is a safety net for OpenSSL developer!

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9133)

Configure

@@ -2209,6 +2209,34 @@ They are ignored and should be replaced with a combination of GENERATE,
 DEPEND and SHARED_SOURCE.
 EOF
 
+
+    # Go through the sources of all libraries and check that the same basename
+    # doesn't appear more than once.  Some static library archivers depend on
+    # them being unique.
+    {
+        my $err = 0;
+        foreach my $prod (keys %{$unified_info{libraries}}) {
+            my @prod_sources =
+                map { keys %{$unified_info{sources}->{$_}} }
+                keys %{$unified_info{sources}->{$prod}};
+            my %srccnt = ();
+
+            # Count how many times a given each source basename
+            # appears for each product.
+            foreach my $src (@prod_sources) {
+                $srccnt{basename $src}++;
+            }
+
+            foreach my $src (keys %srccnt) {
+                if ((my $cnt = $srccnt{$src}) > 1) {
+                    print STDERR "$src appears $cnt times for the product $prod\n";
+                    $err++
+                }
+            }
+        }
+        die if $err > 0;
+    }
+
     # Massage the result
 
     # If we depend on a header file or a perl module, add an inclusion of
@@ -2319,6 +2347,7 @@ EOF
             }
         }
     }
+
     # At this point, we have a number of sources with the value -1.  They
     # aren't part of the local build and are probably meant for a different
     # platform, and can therefore be cleaned away.  That happens when making