Collect common defines into global configuration header

The command lines to pass -D to g++ are quite long (passing in PACKAGE_VERSION, PACKAGE_NAME, etc). They ought to be collected into a common 'config.h' file generated by autoheader:
http://www.gnu.org/software/autoconf/manual/autoconf.html#Configuration-Headers

The #include <config.h> needs to be placed before any other header files, to prevent inconsistencies in declarations (for example, if it redefines const). Plus, not all files may need the include.

This change will likely touch a lot of *.c, *.cc, *.h, and *.i files.