configure --enable-debug changes

Three changes to how configure --enable-debug behaves:

1. Preserve user-passed CXXFLAGS/CFLAGS
2. Compile with -DDEBUG_LOCKORDER
3. Add -DDEBUG -DDEBUG_LOCKORDER to CPPFLAGS (since they are preprocessor options)

Github-Pull: bitcoin#6434
Rebased-From: 83b48c8

configure.ac

@@ -147,12 +147,13 @@ AC_ARG_ENABLE([debug],
     [enable_debug=no])
 
 if test "x$enable_debug" = xyes; then
+    CPPFLAGS="$CPPFLAGS -DDEBUG -DDEBUG_LOCKORDER"
     if test "x$GCC" = xyes; then
-        CFLAGS="-g3 -O0 -DDEBUG"
+        CFLAGS="$CFLAGS -g3 -O0"
     fi
 
     if test "x$GXX" = xyes; then
-        CXXFLAGS="-g3 -O0 -DDEBUG"
+        CXXFLAGS="$CXXFLAGS -g3 -O0"
     fi
 fi