Linux build bug fixes

I've been trying to build cbitcoin on 32 bit Arch Linux, and found a few places where some quick fixes solved issues I was having.

configure.am: Currently, if -arch is unsupported (as it is under Linux), and if -m64 is unsupported, configure will fail. However, there's no reason I can see why 32 bit builds shouldn't work. The change here will still add -m64 to CFLAGS if it's supported, but simply leave it out otherwise without crashing configure.

Makefile.in: Running make was not properly reaching the build target before the core object compilation target, so the build directory was not being created. I moved the prerequisite to the same target as the compilation, which resolved the issue for me, but should not cause problems on other systems. I noticed a similar report in an issue, so maybe there's some differences in make versions.

With these changes, I was able to build the shared libraries (though make test is failing, probably due to dynamic library naming differences between Mac and Linux; I might open an issue to discuss this later). As a note, I haven't committed the new configure script. I thought it would be reasonable to have it generated by a computer where the entire build system is known to work.