test: only run valgrind tests if valgrind.h was found

Disable t5000-valgrind.t by default when valgrind/valgrind.h was
not found by ./configure. This means that the valgrind hook to
disable dlclose() for modules is not active, and this has been found
to cause false positives for this test.

The test can still be forced by hand with the use of -d, --debug flag,
e.g.

 ./t5000-valgrind.t -d

Fixes flux-framework#1097

t/t5000-valgrind.t

@@ -11,6 +11,18 @@ if ! which valgrind >/dev/null; then
     test_done
 fi
 
+# Do not run test by default unless valgrind/valgrind.h was found, since
+#  this has been known to introduce false positives (#1097). However, allow
+#  run to be forced on the cmdline with -d, --debug.
+#
+have_valgrind_h() {
+    grep -q "^#define HAVE_VALGRIND_VALGRIND_H" ${FLUX_BUILD_DIR}/config/config.h
+}
+if ! have_valgrind_h && test "$debug" = ""; then
+    skip_all='skipping valgrind tests b/c valgrind.h not found. Use -d, --debug to force'
+    test_done
+fi
+
 export FLUX_PMI_SINGLETON=1 # avoid finding leaks in slurm libpmi.so
 
 VALGRIND=`which valgrind`