BUILD_CC and BUILD_CXX can be overridden on a per recipe context now so
these tests don't make sense in this form any more. They assume gcc/g++
so lets just use those directly.
This fixes things to work with toolchain selection for the native
case.
(From OE-Core rev: cc93781667cf4f6971bcb9319ab8ab21320852de)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>