build-sys: make libatasmart easily cross-compilable

build-sys: make libatasmart easily cross-compilable

Pick up the AX_CC_FOR_BUILD macro from autoconf-archive (improved, going to
be sent there soon), then use that one to find the host's compiler.

Build the strpool in a different directory, since you cannot override CC
and other variables on a per-target basis yet. While this adds recursivity
to the build system is the best hope you have to properly solve the
cross-compilation.

Update .gitignore accordingly to the above changes, and make sure
bootstrap.sh search macro files in the m4 directory.