Makefile: ensure CONFIG_SITE isn't set

Fixes lib/lib64 issue when building on OpenSUSE.

Reported-by: Guillaume Gardet <guillaume.gardet@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -192,6 +192,7 @@ unexport CPP
 unexport CFLAGS
 unexport CXXFLAGS
 unexport GREP_OPTIONS
+unexport CONFIG_SITE
 
 GNU_HOST_NAME:=$(shell support/gnuconfig/config.guess)