Skip gdbserver tests if ./configure did not find gdb.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11752 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/gdbserver_tests/mcvabits.vgtest b/gdbserver_tests/mcvabits.vgtest
index 689c26f..48b35e8 100644
--- a/gdbserver_tests/mcvabits.vgtest
+++ b/gdbserver_tests/mcvabits.vgtest
@@ -1,4 +1,5 @@
 # test the memcheck V and A bits monitor functionality
+prereq: test -e gdb
 prog: t
 vgopts: --tool=memcheck --vgdb=yes --vgdb-error=0 --vgdb-prefix=./vgdb-prefix-mcvabits
 stdout_filter: filter_make_empty