Fix the misapplied CXXFLAGS.

-Os is a compiler flag, it doesn't belong in the call to the linker. Likewise, -s is a linker flag, it doesn't belong in the call to the compiler.

Clang 3.4 apparently now considers misapplied flags to be worthy of error status when -Werror is applied. Solution: don't give CXXFLAGS to LD, nor LDFLAGS to CXX. Note that most of this was already correct.