fix clang on travis

Ever since 670a6a14a0369e48ddaefbf759eba5a652a99453, picking up the settings of `CC` and `CXX` from environment variables no longer works. I think this is caused by always executing `CC += -m$(BINARY)`. So for the last month or so, Travis hasn't actually been building with clang at all.

An alternate thing we could do is change https://github.com/JuliaLang/julia/blob/542b013eb6dd08480ee11da1db2449d13bfa39e2/Make.inc#L246-L247 to use `?=`, since there are other use cases where you might want to set `CC` and `CXX` via environment variables instead of through `Make.user`, and expect Julia to honor those settings. If you make this change, since `Make.inc` gets included several times by different levels of recursive make, you end up with `-m64` getting repeated 4 times in a bunch of places. It's a little annoying, but probably harmless?