gcc mode: error in case multiple files are given with -c and -o

regression/goto-gcc/multiple-sources/error.desc

@@ -0,0 +1,7 @@
+CORE
+
+-c main1.c main2.c -o outfile
+^EXIT=1$
+^SIGNAL=0$
+^cannot specify -o with -c with multiple files$
+--

regression/goto-gcc/multiple-sources/main1.c

@@ -0,0 +1 @@
+// blank

regression/goto-gcc/multiple-sources/main2.c

@@ -0,0 +1 @@
+// blank

src/goto-cc/gcc_mode.cpp

@@ -723,6 +723,14 @@ int gcc_modet::doit()
     }
   }
 
+  if(
+    cmdline.isset('o') && cmdline.isset('c') &&
+    compiler.source_files.size() >= 2)
+  {
+    error() << "cannot specify -o with -c with multiple files" << eom;
+    return 1; // to match gcc's behaviour
+  }
+
   // Revert to gcc in case there is no source to compile
   // and no binary to link.