configure: Fix valgrind check to take effect, and to work.

We were missing an "override" directive in the assignment of CFLAGS
within Makefile.config so it was actually having no effect. Then, we
were also failing to get the proper include path for valgrind.h so
it wouldn't have worked even it were having effect. Fix both problems.

configure

@@ -61,10 +61,12 @@ fi
 printf "Checking for valgrind development files... "
 if pkg-config --modversion valgrind > /dev/null 2>&1; then
     printf "Yes.\n"
-    have_valgrind=-DHAVE_VALGRIND
+    have_valgrind=1
+    valgrind_flags=$(pkg-config --cflags valgrind)
 else
     printf "No (but that's fine).\n"
-    have_valgrind=
+    have_valgrind=0
+    valgrind_flags=
 fi
 
 if [ $errors -gt 0 ]; then
@@ -137,5 +139,5 @@ EOF
 # construct the Makefile.config
 cat > Makefile.config <<EOF
 prefix = /usr/local
-CFLAGS += ${have_valgrind}
+override CFLAGS += -DHAVE_VALGRIND=${have_valgrind} ${valgrind_flags}
 EOF