scripts: unset CONFIG_SITE

Some distributions (eg. openSUSE 12.1) systematically export
the CONFIG_SITE environment variable to point to a custom
script setting misc paths for ./configure.

This can, and does, break when cross-compiling for architectures
that are not supported by this script.

The simple workaround is to unset this variable.
NB: buildroot has a similar fix:
http://git.buildroot.org/buildroot/commit/?id=12c9f7dd6dee9c6029b4f9a12d6aac1516911ab4

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 21f4f28e60ec0342133086c9a51e7f0e5b181fb8)

 Fix for psim build failure due to -lz

 See http://sourceware.org/bugzilla/show_bug.cgi?id=12202

 --- gdb-7.2.orig/sim/ppc/Makefile.in

 +++ gdb-7.2/sim/ppc/Makefile.in

 @@ -551,7 +551,7 @@

  PACKAGE_OBJ = @sim_pk_obj@

 -psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBS) $(LIBINTL_DEP)

 +psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBINTL_DEP)

  	$(CC) $(CFLAGS) $(SIM_CFLAGS) $(LDFLAGS) -o psim$(EXEEXT) main.o $(TARGETLIB) $(BFD_LIB) $(LIBINTL) $(LIBIBERTY_LIB) $(LIBS)

  run: psim