Linux: not cross-compilable to x86 on 64bit

[hideki]

clang does not work with gcc-multilib and g++multilib. It is not cross-compilable to x86 on 64bit machine

[pasin]

@hideki does this only affect x86 binary?

[hideki]

As we build forestdb binary on x86 (64bit) with clang, we can not compile forestdb binary to x86 (32bit).

If we can use GCC for forestdb, we could avoid this. Or we should setup x86 (32bit) build environment for forestdb.