#6 Ignore /usr/local by default

[Christian Biere]

Configure uses -I/usr/local/include and -L/usr/local/lib
by default. I don't even mind if that's the default but
there should be a simple way to prevent this like
-Diamsmarterthanthou because this could cause unintented
compilation against a certain or simply the
wrong libraries. If Configure looks in /usr/local there
isn't much of a reason not to look in every available
path, is there? Furthermore, without
-Wl,-R/usr/local/lib (or whatever flag the compiler/link
wants) this will likely cause a broken executable
because it won't find the dynamically linked libraries
it might have been linked against.