bash: remove CC variable when calling make

So "/opt/whatever_toolchain/gcc --sysroot $(STAGING_DIR)" isn't
stripped to "/opt/whatever_toolchain/gcc", making gcc to look at
$(ORIGINAL_TOOLCHAIN_DIRECTORY)/usr/include instead of
$(STAGING_DIR)/usr/include. For more CC is already filled when
running the configure script.

This problem should only be visible with an external toolchain.

Signed-off-by: Lionel Landwerlin <llandwerlin@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>