Enhance gcc test for visible variables with -fwhole-program.

Add 'extern' decl before variable test - some older compilers wont
    mark a variable visible if an extern decl precedes it.
diff --git a/tools/test-gcc.sh b/tools/test-gcc.sh
index 5f55f37..738cf64 100755
--- a/tools/test-gcc.sh
+++ b/tools/test-gcc.sh
@@ -19,6 +19,7 @@
 # Test if "visible" variables and functions are marked global.
 cat - > $TMPFILE1 <<EOF
 void __attribute__((externally_visible)) t1() { }
+extern unsigned char v1;
 unsigned char v1 __attribute__((section(".data16.foo.19"))) __attribute__((externally_visible));
 EOF
 $CC -Os -c -fwhole-program $TMPFILE1 -o $TMPFILE1o > /dev/null 2>&1