configure: Use the tool specified with --valgrind instead of a hardcoded "valgrind"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -2507,7 +2507,7 @@ case "$toolchain" in
         add_ldflags -fsanitize=thread -pie
     ;;
     valgrind-memcheck)
-        target_exec_default="valgrind"
+        target_exec_default=${valgrind:-"valgrind"}
         target_exec_args="--error-exitcode=1 --malloc-fill=0x2a --track-origins=yes --leak-check=full --gen-suppressions=all --suppressions=$source_path/tests/fate-valgrind.supp"
     ;;
     msvc)