Switch build system to autoconf + buildsys.mk

This greatly increases portability, as platform defines are replaced
with feature checks. It can already build namecoind, though the checks
in configure.ac could be expanded a little.

Support for Namecoin-Qt is completely missing, but will be added
shortly.
Include config.h in every source file

This is a requirement from autoconf.
configure: Add option for miniupnpc
Check for Boost headers

This is useful in case someone installed the Boost libs without the
headers.
configure: Don't check for a C compiler for CPP

This uses the C++ precompiler for CPP instead.
Remove makefile.mingw
Build Namecoin-Qt using new build system
configure: Check for $tool-qt4 before $tool

ArchLinux adds the -qt4 suffix for the tools from Qt4 to avoid conflicts
with Qt5, so it's better to try those first.
Fix compilation on Linux