Add knob to disable valgrind at test runtime.

When valgrind is installed in system but is not
working we should still be able to run tests.

Bug: https://bugs.gentoo.org/424377
--- a/waftools/unittest.py
+++ b/waftools/unittest.py
@@ -64,7 +64,8 @@ def generate_coverage(bld):
 
 def configure(conf):
     conf.load("waf_unit_test")
-    conf.find_program("valgrind", var="VALGRIND", mandatory=False)
+    if conf.options.enable_valgrind is True:
+        conf.find_program("valgrind", var="VALGRIND", mandatory=False)
     conf.find_program("lcov", var="LCOV", mandatory=False)
     conf.find_program("genhtml", var="GENHTML", mandatory=False)
 
--- a/wscript
+++ b/wscript
@@ -501,6 +501,10 @@ def options(opt):
                    dest='ldconfig', help="Run ldconfig after install even if not root")
     opt.add_option('--without-ldconfig', action='store_false',
                    dest='ldconfig', help="Don't run ldconfig after install")
+    opt.add_option('--with-valgrind', action='store_true', default=None,
+                   dest='enable_valgrind', help="Run testsuite under valgrind (if present).")
+    opt.add_option('--without-valgrind', action='store_false', default=None,
+                   dest='enable_valgrind', help="Don't run testsuite under valgrind.")
 
     opt.sub_options("src/xmms")
     for o in optional_subdirs + subdirs: